Method and user device for receiving uplink control information, and method and base station for transmitting uplink control information

ABSTRACT

The base station of the present invention notifies a user device of a first set of cells and a set of parameters corresponding respectively to the cells of the first set of cells, and notifies the user device of a second set of cells that is a subset of the first set of cells and that is associated with the user device. The user device may receive a downlink signal or transmit an uplink signal by sing one of the sets of parameters associated with the second set of cells.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the National Phase of PCT/KR2013/011728 filed onDec. 17, 2013, which claims priority under 35 U.S.C. 119(e) to U.S.Provisional Application No. 61/738,392 filed on Dec. 17, 2012, all ofwhich are hereby expressly incorporated by reference into the presentapplication.

TECHNICAL FIELD

The present invention relates to a wireless communication system and,more particularly, to a method for transmitting or receiving a uplink ordownlink signal and an apparatus therefor.

BACKGROUND ART

With appearance and spread of machine-to-machine (M2M) communication anda variety of devices such as smartphones and tablet PCs and technologydemanding a large amount of data transmission, data throughput needed ina cellular network has rapidly increased. To satisfy such rapidlyincreasing data throughput, carrier aggregation technology, cognitiveradio technology, etc. for efficiently employing more frequency bandsand multiple input multiple output (MIMO) technology, multi-base station(BS) cooperation technology, etc. for raising data capacity transmittedon limited frequency resources have been developed.

A general wireless communication system performs datatransmission/reception through one downlink (DL) band and through oneuplink (UL) band corresponding to the DL band (in case of a frequencydivision duplex (FDD) mode), or divides a prescribed radio frame into aUL time unit and a DL time unit in the time domain and then performsdata transmission/reception through the UL/DL time unit (in case of atime division duplex (TDD) mode). A base station (BS) and a userequipment (UE) transmit and receive data and/or control informationscheduled on a prescribed time unit basis, e.g. on a subframe basis. Thedata is transmitted and received through a data region configured in aUL/DL subframe and the control information is transmitted and receivedthrough a control region configured in the UL/DL subframe. To this end,various physical channels carrying radio signals are formed in the UL/DLsubframe. In contrast, carrier aggregation technology serves to use awider UL/DL bandwidth by aggregating a plurality of UL/DL frequencyblocks in order to use a broader frequency band so that more signalsrelative to signals when a single carrier is used can be simultaneouslyprocessed.

In addition, a communication environment has evolved into increasingdensity of nodes accessible by a user at the periphery of the nodes. Anode refers to a fixed point capable of transmitting/receiving a radiosignal to/from the UE through one or more antennas. A communicationsystem including high-density nodes may provide a better communicationservice to the UE through cooperation between the nodes.

DETAILED DESCRIPTION OF THE INVENTION Technical Problems

Due to introduction of new radio communication technology, the number ofuser equipments (UEs) to which a BS should provide a service in aprescribed resource region increases and the amount of data and controlinformation that the BS should transmit to the UEs increases. Since theamount of resources available to the BS for communication with the UE(s)is limited, a new method in which the BS efficiently receives/transmitsuplink/downlink data and/or uplink/downlink control information usingthe limited radio resources is needed.

In addition, a method for efficiently transmitting/receiving, on limitedradio resources, a reference signal, which is used when a control signaland/or a data signal transmitted by a transmitting device is restored bya receiving device is also demanded.

The technical objects that can be achieved through the present inventionare not limited to what has been particularly described hereinabove andother technical objects not described herein will be more clearlyunderstood by persons skilled in the art from the following detaileddescription.

Technical Solutions

A base station of the present invention informs a user equipment of afirst cell set, parameter sets corresponding to cells of the first cellset respectively, and a second cell set which is a subset of the firstcell set and associated with the user equipment. The user equipment mayreceive a downlink signal or transmit an uplink signal using one of theparameter sets related to the second cell set.

The object of the present invention can be achieved by providing amethod for receiving a downlink signal in a user equipment, the methodincluding receiving first information on a first cell set including aplurality of cells and a plurality of parameter sets corresponding tothe cells respectively, receiving second information indicating a secondcell set including at least one of the cells, receiving thirdinformation indicating a specific cell in the second cell set, andreceiving the downlink signal through the specific cell using aparameter set of the specific cell among the parameter sets based on thethird information.

In another aspect of the present invention, provided herein is a userequipment for receiving a downlink signal, the user equipment includinga radio frequency (RF) unit and a processor configured to control the RFunit, wherein the processor controls the RF unit to receive firstinformation on a first cell set including a plurality of cells and aplurality of parameter sets corresponding to the cells respectively,second information indicating a second cell set including at least oneof the cells, and third information indicating a specific cell in thesecond cell set, and to receive the downlink signal through the specificcell using a parameter set of the specific cell among the parameter setsbased on the third information.

In another aspect of the present invention, provided herein is a methodfor transmitting a downlink signal in a base station, the methodincluding transmitting first information on a first cell set including aplurality of cells and a plurality of parameter sets corresponding tothe cells respectively, transmitting second information indicating asecond cell set including at least one of the cells, transmitting thirdinformation indicating a specific cell in the second cell set, andtransmitting the downlink signal through the specific cell using aparameter set of the specific cell among the parameter sets based on thethird information.

In another aspect of the present invention, provided herein is a basestation for transmitting a downlink signal, the base station including aradio frequency (RF) unit and a processor configured to control the RFunit, wherein the processor controls the RF unit to transmit firstinformation on a first cell set including a plurality of cells and aplurality of parameter sets corresponding to the cells respectively,second information indicating a second cell set including at least oneof the cells, and third information indicating a specific cell in thesecond cell set, and to transmit the downlink signal through thespecific cell using a parameter set of the specific cell among theparameter sets based on the third information.

In each of the aspects of the present invention, each of the parametersets may include at least the number of antenna ports of a correspondingcell, zero power CSI-RS (channel state information reference signal)resource configuration information of the corresponding cell,information indicating a start symbol of a physical downlink controlchannel of the corresponding cell or non-zero power CSI-RS resourceconfiguration information of the corresponding cell.

In each of the aspects of the present invention, fourth informationindicating switching of a serving cell to another cell different fromthe specific cell may be further received through the physical downlinkcontrol channel of the specific cell.

In each of the aspects of the present invention, another signal may bereceived or transmitted through the another cell using a parameter setof the another cell among the parameter sets based on the fourthinformation.

In each of the aspects of the present invention, the another cell maybelong to the second cell set.

In each of the aspects of the present invention, decoding of a discoverysignal for each cell in the second cell set may be attempted todetermine a state of each cell in the second cell set.

The above technical solutions are merely some parts of the embodimentsof the present invention and various embodiments into which thetechnical features of the present invention are incorporated can bederived and understood by persons skilled in the art from the followingdetailed description of the present invention.

Advantageous Effect

According to embodiments of the present invention, uplink/downlinksignals may be efficiently transmitted/received. Accordingly, overallthroughput of a wireless communication system is enhanced.

According to embodiments of the present invention, a UE may efficientlyperform handover.

It will be appreciated by persons skilled in the art that that theeffects that can be achieved through the present invention are notlimited to what has been particularly described hereinabove and otheradvantages of the present invention will be more clearly understood fromthe following detailed description.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

FIG. 1 illustrates the structure of a radio frame used in a wirelesscommunication system.

FIG. 2 illustrates the structure of a downlink (DL)/uplink (UL) slot ina wireless communication system.

FIG. 3 illustrates the structure of a DL subframe used in a wirelesscommunication system.

FIG. 4 illustrates the structure of a UL subframe used in a wirelesscommunication system.

FIG. 5 is a diagram for explaining single-carrier communication andmulti-carrier communication.

FIG. 6 illustrates a physical downlink control channel (PDCCH) or anenhanced PDCCH (EPDCCH), and a data channel scheduled by PDCCH/EPDCCH.

FIG. 7 illustrates configuration of cell specific reference signals(CRSs) and user specific reference signals (UE-RS).

FIG. 8 illustrates channel state information reference signal (CSI-RS)configurations.

FIG. 9 illustrates the concept of a small cell.

FIG. 10 illustrates the conventional handover process.

FIG. 11 is a diagram illustrating cell sets according to one embodimentof the present invention.

FIGS. 12 and 13 are diagrams illustrating parameter set(s) according toone embodiment of the present invention.

FIG. 14 is a diagram illustrating exemplary transmission of a discoverysignal.

FIG. 15 is a block diagram illustrating elements of a transmittingdevice 10 and a receiving device 20 for implementing the presentinvention.

MODE FOR CARRYING OUT THE INVENTION

Reference will now be made in detail to the exemplary embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. The detailed description, which will be given below withreference to the accompanying drawings, is intended to explain exemplaryembodiments of the present invention, rather than to show the onlyembodiments that can be implemented according to the invention. Thefollowing detailed description includes specific details in order toprovide a thorough understanding of the present invention. However, itwill be apparent to those skilled in the art that the present inventionmay be practiced without such specific details.

In some instances, known structures and devices are omitted or are shownin block diagram form, focusing on important features of the structuresand devices, so as not to obscure the concept of the present invention.The same reference numbers will be used throughout this specification torefer to the same or like parts.

The following techniques, apparatuses, and systems may be applied to avariety of wireless multiple access systems. Examples of the multipleaccess systems include a code division multiple access (CDMA) system, afrequency division multiple access (FDMA) system, a time divisionmultiple access (TDMA) system, an orthogonal frequency division multipleaccess (OFDMA) system, a single carrier frequency division multipleaccess (SC-FDMA) system, and a multicarrier frequency division multipleaccess (MC-FDMA) system. CDMA may be embodied through radio technologysuch as universal terrestrial radio access (UTRA) or CDMA2000. TDMA maybe embodied through radio technology such as global system for mobilecommunications (GSM), general packet radio service (GPRS), or enhanceddata rates for GSM evolution (EDGE). OFDMA may be embodied through radiotechnology such as institute of electrical and electronics engineers(IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, or evolved UTRA(E-UTRA). UTRA is a part of a universal mobile telecommunications system(UMTS). 3rd generation partnership project (3GPP) long term evolution(LTE) is a part of evolved UMTS (E-UMTS) using E-UTRA. 3GPP LTE employsOFDMA in DL and SC-FDMA in UL. LTE-advanced (LTE-A) is an evolvedversion of 3GPP LTE. For convenience of description, it is assumed thatthe present invention is applied to 3GPP LTE/LTE-A. However, thetechnical features of the present invention are not limited thereto. Forexample, although the following detailed description is given based on amobile communication system corresponding to a 3GPP LTE/LTE-A system,aspects of the present invention that are not specific to 3GPP LTE/LTE-Aare applicable to other mobile communication systems.

For example, the present invention is applicable to contention basedcommunication such as Wi-Fi as well as non-contention basedcommunication as in the 3GPP LTE/LTE-A system in which an eNB allocatesa DL/UL time/frequency resource to a UE and the UE receives a DL signaland transmits a UL signal according to resource allocation of the eNB.In a non-contention based communication scheme, an access point (AP) ora control node for controlling the AP allocates a resource forcommunication between the UE and the AP, whereas, in a contention basedcommunication scheme, a communication resource is occupied throughcontention between UEs which desire to access the AP. The contentionbased communication scheme will now be described in brief. One type ofthe contention based communication scheme is carrier sense multipleaccess (CSMA). CSMA refers to a probabilistic media access control (MAC)protocol for confirming, before a node or a communication devicetransmits traffic on a shared transmission medium (also called a sharedchannel) such as a frequency band, that there is no other traffic on thesame shared transmission medium. In CSMA, a transmitting devicedetermines whether another transmission is being performed beforeattempting to transmit traffic to a receiving device. In other words,the transmitting device attempts to detect presence of a carrier fromanother transmitting device before attempting to perform transmission.Upon sensing the carrier, the transmitting device waits for anothertransmission device which is performing transmission to finishtransmission, before performing transmission thereof. Consequently, CSMAcan be a communication scheme based on the principle of “sense beforetransmit” or “listen before talk”. A scheme for avoiding collisionbetween transmitting devices in the contention based communicationsystem using CSMA includes carrier sense multiple access with collisiondetection (CSMA/CD) and/or carrier sense multiple access with collisionavoidance (CSMA/CA). CSMA/CD is a collision detection scheme in a wiredlocal area network (LAN) environment. In CSMA/CD, a personal computer(PC) or a server which desires to perform communication in an Ethernetenvironment first confirms whether communication occurs on a networkand, if another device carries data on the network, the PC or the serverwaits and then transmits data. That is, when two or more users (e.g.PCs, UEs, etc.) simultaneously transmit data, collision occurs betweensimultaneous transmission and CSMA/CD is a scheme for flexiblytransmitting data by monitoring collision. A transmitting device usingCSMA/CD adjusts data transmission thereof by sensing data transmissionperformed by another device using a specific rule. CSMA/CA is a MACprotocol specified in IEEE 802.11 standards. A wireless LAN (WLAN)system conforming to IEEE 802.11 standards does not use CSMA/CD whichhas been used in IEEE 802.3 standards and uses CA, i.e. a collisionavoidance scheme. Transmission devices always sense carrier of a networkand, if the network is empty, the transmission devices wait fordetermined time according to locations thereof registered in a list andthen transmit data. Various methods are used to determine priority ofthe transmission devices in the list and to reconfigure priority. In asystem according to some versions of IEEE 802.11 standards, collisionmay occur and, in this case, a collision sensing procedure is performed.A transmission device using CSMA/CA avoids collision between datatransmission thereof and data transmission of another transmissiondevice using a specific rule.

In the present invention, a user equipment (UE) may be a fixed or mobiledevice. Examples of the UE include various devices that transmit andreceive user data and/or various kinds of control information to andfrom a base station (BS). The UE may be referred to as a terminalequipment (TE), a mobile station (MS), a mobile terminal (MT), a userterminal (UT), a subscriber station (SS), a wireless device, a personaldigital assistant (PDA), a wireless modem, a handheld device, etc. Inaddition, in the present invention, a BS generally refers to a fixedstation that performs communication with a UE and/or another BS, andexchanges various kinds of data and control information with the UE andanother BS. The BS may be referred to as an advanced base station (ABS),a node-B (NB), an evolved node-B (eNB), a base transceiver system (BTS),an access point (AP), a processing server (PS), etc. In describing thepresent invention, a BS will be referred to as an eNB.

In the present invention, a node refers to a fixed point capable oftransmitting/receiving a radio signal through communication with a UE.Various types of eNBs may be used as nodes irrespective of the termsthereof. For example, a BS, a node B (NB), an e-node B (eNB), apico-cell eNB (PeNB), a home eNB (HeNB), a relay, a repeater, etc. maybe a node. In addition, the node may not be an eNB. For example, thenode may be a radio remote head (RRH) or a radio remote unit (RRU). TheRRH or RRU generally has a lower power level than a power level of aneNB. Since the RRH or RRU (hereinafter, RRH/RRU) is generally connectedto the eNB through a dedicated line such as an optical cable,cooperative communication between RRH/RRU and the eNB can be smoothlyperformed in comparison with cooperative communication between eNBsconnected by a radio line. At least one antenna is installed per node.The antenna may mean a physical antenna or mean an antenna port, avirtual antenna, or an antenna group. A node may be referred to as apoint. In the multi-node system, the same cell identity (ID) ordifferent cell IDs may be used to transmit/receive signals to/from aplurality of nodes. If the plural nodes have the same cell ID, each ofthe nodes operates as a partial antenna group of one cell. If the nodeshave different cell IDs in the multi-node system, the multi-node systemmay be regarded as a multi-cell (e.g. a macro-cell/femto-cell/pico-cell)system. If multiple cells formed respectively by multiple nodes areconfigured in an overlaid form according to coverage, a network formedby the multiple cells is referred to as a multi-tier network. A cell IDof an RRH/RRU may be the same as or different from a cell ID of an eNB.When the RRH/RRU and the eNB use different cell IDs, both the RRH/RRUand the eNB operate as independent eNBs.

In the multi-node system, one or more eNBs or eNB controllers connectedto multiple nodes may control the nodes such that signals aresimultaneously transmitted to or received from a UE through some or allnodes. While there is a difference between multi-node systems accordingto the nature of each node and implementation form of each node,multi-node systems are discriminated from single node systems (e.g. acentralized antenna system (CAS), conventional MIMO systems,conventional relay systems, conventional repeater systems, etc.) since aplurality of nodes provides communication services to a UE in apredetermined time-frequency resource. Accordingly, embodiments of thepresent invention with respect to a method of performing coordinateddata transmission using some or all nodes may be applied to varioustypes of multi-node systems. For example, a node refers to an antennagroup spaced apart from another node by a predetermined distance ormore, in general. However, embodiments of the present invention, whichwill be described below, may even be applied to a case in which a noderefers to an arbitrary antenna group irrespective of node interval. Inthe case of an eNB including an X-pole (cross-polarized) antenna, forexample, the embodiments of the preset invention are applicable on theassumption that the eNB controls a node composed of an H-pole antennaand a node composed of a V-pole antenna.

A communication scheme through which signals are transmitted/receivedvia a plurality of transmit (Tx)/receive (Rx) nodes, signals aretransmitted/received via at least one node selected from a plurality ofTx/Rx nodes, or a node transmitting a DL signal is discriminated from anode transmitting a UL signal is called multi-eNB MIMO or coordinatedmulti-point transmission/reception (CoMP). Coordinated transmissionschemes from among CoMP communication schemes may be broadly categorizedinto joint processing (JP) and scheduling coordination. The former maybe divided into joint transmission (JT)/joint reception (JR) and dynamicpoint selection (DPS) and the latter may be divided into coordinatedscheduling (CS) and coordinated beamforming (CB). DPS may be calleddynamic cell selection (DCS). When JP is performed, a wider variety ofcommunication environments can be formed, compared to other CoMPschemes. JT refers to a communication scheme by which a plurality ofnodes transmits the same stream to a UE and JR refers to a communicationscheme by which a plurality of nodes receive the same stream from theUE. The UE/eNB combine signals received from the plurality of nodes torestore the stream. In the case of JT/JR, signal transmissionreliability can be improved according to transmit diversity since thesame stream is transmitted to/from a plurality of nodes. In JP, DPSrefers to a communication scheme by which a signal istransmitted/received through a node selected from a plurality of nodesaccording to a specific rule. In the case of DPS, signal transmissionreliability can be improved because a node having a good channel statebetween the node and the UE is selected as a communication node.

In the present invention, a cell refers to a prescribed geographicalarea to which one or more nodes provide a communication service.Accordingly, in the present invention, communicating with a specificcell may mean communicating with an eNB or a node which provides acommunication service to the specific cell. In addition, a DL/UL signalof a specific cell refers to a DL/UL signal from/to an eNB or a nodewhich provides a communication service to the specific cell. A nodeproviding UL/DL communication services to a UE is called a serving nodeand a cell to which UL/DL communication services are provided by theserving node is especially called a serving cell. Furthermore, channelstatus/quality of a specific cell refers to channel status/quality of achannel or communication link formed between an eNB or node whichprovides a communication service to the specific cell and a UE. The UEmay measure DL channel state received from a specific node usingcell-specific reference signal(s) (CRS(s)) transmitted on a CRS resourceand/or channel state information reference signal(s) (CSI-RS(s))transmitted on a CSI-RS resource, allocated by antenna port(s) of thespecific node to the specific node. Meanwhile, a 3GPP LTE/LTE-A systemuses the concept of a cell in order to manage radio resources and a cellassociated with the radio resources is distinguished from a cell of ageographic region.

A “cell” of a geographic region may be understood as coverage withinwhich a node can provide a service using a carrier and a “cell” of aradio resource is associated with bandwidth (BW) which is a frequencyrange configured by the carrier. Since DL coverage, which is a rangewithin which the node is capable of transmitting a valid signal, and ULcoverage, which is a range within which the node is capable of receivingthe valid signal from the UE, depends upon a carrier carrying thesignal, coverage of the node may be associated with coverage of “cell”of a radio resource used by the node. Accordingly, the term “cell” maybe used to indicate service coverage by the node sometimes, a radioresource at other times, or a range that a signal using a radio resourcecan reach with valid strength at other times. The “cell” of the radioresource will be described in detail when carrier aggregation isdescribed.

3GPP LTE/LTE-A standards define DL physical channels corresponding toresource elements carrying information derived from a higher layer andDL physical signals corresponding to resource elements which are used bya physical layer but which do not carry information derived from ahigher layer. For example, a physical downlink shared channel (PDSCH), aphysical broadcast channel (PBCH), a physical multicast channel (PMCH),a physical control format indicator channel (PCFICH), a physicaldownlink control channel (PDCCH), and a physical hybrid ARQ indicatorchannel (PHICH) are defined as the DL physical channels, and a referencesignal and a synchronization signal are defined as the DL physicalsignals. A reference signal (RS), also called a pilot, refers to aspecial waveform of a predefined signal known to both a BS and a UE. Forexample, a cell-specific RS (CRS), a UE-specific RS (UE-RS), apositioning RS (PRS), and channel state information RS (CSI-RS) may bedefined as DL RSs. Meanwhile, the 3GPP LTE/LTE-A standards define ULphysical channels corresponding to resource elements carryinginformation derived from a higher layer and UL physical signalscorresponding to resource elements which are used by a physical layerbut which do not carry information derived from a higher layer. Forexample, a physical uplink shared channel (PUSCH), a physical uplinkcontrol channel (PUCCH), and a physical random access channel (PRACH)are defined as the UL physical channels, and a demodulation referencesignal (DM RS) for a UL control/data signal and a sounding referencesignal (SRS) used for UL channel measurement are defined as the ULphysical signals.

In the present invention, a physical downlink control channel (PDCCH), aphysical control format indicator channel (PCFICH), a physical hybridautomatic retransmit request indicator channel (PHICH), and a physicaldownlink shared channel (PDSCH) refer to a set of time-frequencyresources or resource elements (REs) carrying downlink controlinformation (DCI), a set of time-frequency resources or REs carrying acontrol format indicator (CFI), a set of time-frequency resources or REscarrying downlink acknowledgement (ACK)/negative ACK (NACK), and a setof time-frequency resources or REs carrying downlink data, respectively.In addition, a physical uplink control channel (PUCCH), a physicaluplink shared channel (PUSCH) and a physical random access channel(PRACH) refer to a set of time-frequency resources or REs carryinguplink control information (UCI), a set of time-frequency resources orREs carrying uplink data and a set of time-frequency resources or REscarrying random access signals, respectively. In the present invention,in particular, a time-frequency resource or RE that is assigned to orbelongs to PDCCH/PCFICH/PHICH/PDSCH/PUCCH/PUSCH/PRACH is referred to asPDCCH/PCFICH/PHICH/PDSCH/PUCCH/PUSCH/PRACH RE orPDCCH/PCFICH/PHICH/PDSCH/PUCCH/PUSCH/PRACH time-frequency resource,respectively. Therefore, in the present invention, PUCCH/PUSCH/PRACHtransmission of a UE is conceptually identical to UCI/uplink data/randomaccess signal transmission on PUSCH/PUCCH/PRACH, respectively. Inaddition, PDCCH/PCFICH/PHICH/PDSCH transmission of an eNB isconceptually identical to downlink data/DCI transmission onPDCCH/PCFICH/PHICH/PDSCH, respectively.

In addition, in the present invention, a PBCH/(e)PDCCH/PDSCH/PUCCH/PUSCHregion refers to a time-frequency resource region to whichPBCH/(e)PDCCH/PDSCH/PUCCH/PUSCH has been mapped or may be mapped.

Hereinafter, OFDM symbol/subcarrier/RE to or for whichCRS/DMRS/CSI-RS/SRS/UE-RS/TRS is assigned or configured will be referredto as CRS/DMRS/CSI-RS/SRS/UE-RS/TRS symbol/carrier/subcarrier/RE. Forexample, an OFDM symbol to or for which a tracking RS (TRS) is assignedor configured is referred to as a TRS symbol, a subcarrier to or forwhich the TRS is assigned or configured is referred to as a TRSsubcarrier, and an RE to or for which the TRS is assigned or configuredis referred to as a TRS RE. In addition, a subframe configured fortransmission of the TRS is referred to as a TRS subframe. Moreover, asubframe in which a broadcast signal is transmitted is referred to as abroadcast subframe or a PBCH subframe and a subframe in which asynchronization signal (e.g. PSS and/or SSS) is transmitted is referredto a synchronization signal subframe or a PSS/SSS subframe. OFDMsymbol/subcarrier/RE to or for which PSS/SSS is assigned or configuredis referred to as PSS/SSS symbol/subcarrier/RE, respectively.

In the present invention, a CRS port, a UE-RS port, a CSI-RS port, and aTRS port refer to an antenna port configured to transmit a CRS, anantenna port configured to transmit a UE-RS, an antenna port configuredto transmit a CSI-RS, and an antenna port configured to transmit a TRS,respectively. Antenna ports configured to transmit CRSs may bedistinguished from each other by the locations of REs occupied by theCRSs according to CRS ports, antenna ports configured to transmit UE-RSsmay be distinguished from each other by the locations of REs occupied bythe UE-RSs according to UE-RS ports, and antenna ports configured totransmit CSI-RSs may be distinguished from each other by the locationsof REs occupied by the CSI-RSs according to CSI-RS ports. Therefore, theterm CRS/UE-RS/CSI-RS/TRS ports may also be used to indicate a patternof REs occupied by CRSs/UE-RSs/CSI-RSs/TRSs in a predetermined resourceregion. In the present invention, both DMRS and UE-RS representdemodulation RSs, and thus the terms DMRS and UE-RS are used to refer todemodulation RSs.

FIG. 1 illustrates the structure of a radio frame used in a wirelesscommunication system.

Specifically, FIG. 1(a) illustrates an exemplary structure of a radioframe which can be used in frequency division multiplexing (FDD) in 3GPPLTE/LTE-A and FIG. 1(b) illustrates an exemplary structure of a radioframe which can be used in time division multiplexing (TDD) in 3GPPLTE/LTE-A.

Referring to FIG. 1, a 3GPP LTE/LTE-A radio frame is 10 ms(307,200T_(s)) in duration. The radio frame is divided into 10 subframesof equal size. Subframe numbers may be assigned to the 10 subframeswithin one radio frame, respectively. Here, T_(s) denotes sampling timewhere T_(s)=1/(2048*15 kHz). Each subframe is 1 ms long and is furtherdivided into two slots. 20 slots are sequentially numbered from 0 to 19in one radio frame. Duration of each slot is 0.5 ms. A time interval inwhich one subframe is transmitted is defined as a transmission timeinterval (TTI). Time resources may be distinguished by a radio framenumber (or radio frame index), a subframe number (or subframe index), aslot number (or slot index), and the like.

A radio frame may have different configurations according to duplexmodes. In FDD mode for example, since DL transmission and ULtransmission are discriminated according to frequency, a radio frame fora specific frequency band operating on a carrier frequency includeseither DL subframes or UL subframes. In TDD mode, since DL transmissionand UL transmission are discriminated according to time, a radio framefor a specific frequency band operating on a carrier frequency includesboth DL subframes and UL subframes.

Table 1 shows an exemplary UL-DL configuration within a radio frame inTDD mode.

TABLE 1 Uplink- Downlink- downlink to-Uplink config- Switch-pointSubframe number uration periodicity 0 1 2 3 4 5 6 7 8 9 0 5 ms D S U U UD S U U U 1 5 ms D S U U D D S U U D 2 5 ms D S U D D D S U D D 3 10 ms D S U U U D D D D D 4 10 ms  D S U U D D D D D D 5 10 ms  D S U D D D DD D D 6 5 ms D S U U U D S U U D

In Table 1, D denotes a DL subframe, U denotes a UL subframe, and Sdenotes a special subframe. The special subframe includes three fields,i.e. downlink pilot time slot (DwPTS), guard period (GP), and uplinkpilot time slot (UpPTS). DwPTS is a time slot reserved for DLtransmission and UpPTS is a time slot reserved for UL transmission.Table 2 shows an example of the special subframe configuration.

TABLE 2 Normal cyclic prefix in downlink Extended cyclic prefix indownlink UpPTS UpPTS Normal Normal Special cyclic Extended cyclicExtended subframe prefix in cyclic prefix prefix in cyclic prefixconfiguration DwPTS uplink in uplink DwPTS uplink in uplink 0  6592 ·T_(s) 2192 · T_(s) 2560 · T_(s)  7680 · T_(s) 2192 · T_(s) 2560 · T_(s)1 19760 · T_(s) 20480 · T_(s) 2 21952 · T_(s) 23040 · T_(s) 3 24144 ·T_(s) 25600 · T_(s) 4 26336 · T_(s)  7680 · T_(s) 4384 · T_(s) 5120 ·T_(s) 5  6592 · T_(s) 4384 · T_(s) 5120 · T_(s) 20480 · T_(s) 6 19760 ·T_(s) 23040 · T_(s) 7 21952 · T_(s) — — — 8 24144 · T_(s) — — —

FIG. 2 illustrates the structure of a DL/UL slot structure in a wirelesscommunication system. In particular, FIG. 2 illustrates the structure ofa resource grid of a 3GPP LTE/LTE-A system. One resource grid is definedper antenna port.

Referring to FIG. 2, a slot includes a plurality of orthogonal frequencydivision multiplexing (OFDM) symbols in the time domain and includes aplurality of resource blocks (RBs) in the frequency domain. The OFDMsymbol may refer to one symbol duration. Referring to FIG. 2, a signaltransmitted in each slot may be expressed by a resource grid includingN^(DL/UL) _(RB)*N^(RB) _(sc) subcarriers and N^(DL/UL) _(symb) OFDMsymbols. N^(DL) _(RB) denotes the number of RBs in a DL slot and N^(UL)_(RB) denotes the number of RBs in a UL slot. N^(DL) _(RB) and N^(UL)_(RB) depend on a DL transmission bandwidth and a UL transmissionbandwidth, respectively. N^(DL) _(symb) denotes the number of OFDMsymbols in a DL slot, N^(UL) _(symb) denotes the number of OFDM symbolsin a UL slot, and N^(RB) _(sc) denotes the number of subcarriersconfiguring one RB.

An OFDM symbol may be referred to as an OFDM symbol, a single carrierfrequency division multiplexing (SC-FDM) symbol, etc. according tomultiple access schemes. The number of OFDM symbols included in one slotmay be varied according to channel bandwidths and CP lengths. Forexample, in a normal cyclic prefix (CP) case, one slot includes 7 OFDMsymbols. In an extended CP case, one slot includes 6 OFDM symbols.Although one slot of a subframe including 7 OFDM symbols is shown inFIG. 2 for convenience of description, embodiments of the presentinvention are similarly applicable to subframes having a differentnumber of OFDM symbols. Referring to FIG. 2, each OFDM symbol includesN^(DL/UL) _(RB)*N^(RB) _(sc) subcarriers in the frequency domain. Thetype of the subcarrier may be divided into a data subcarrier for datatransmission, a reference signal (RS) subcarrier for RS transmission,and a null subcarrier for a guard band and a DC component. The nullsubcarrier for the DC component is unused and is mapped to a carrierfrequency f₀ in a process of generating an OFDM signal or in a frequencyup-conversion process. The carrier frequency is also called a centerfrequency f_(c).

One RB is defined as N^(DL/UL) _(symb) (e.g. 7) consecutive OFDM symbolsin the time domain and as N^(RB) _(sc) (e.g. 12) consecutive subcarriersin the frequency domain. For reference, a resource composed of one OFDMsymbol and one subcarrier is referred to a resource element (RE) ortone. Accordingly, one RB includes N^(DL/UL) _(symb)*N^(RB) _(sc) REs.Each RE within a resource grid may be uniquely defined by an index pair(k, l) within one slot. k is an index ranging from 0 to N^(DL/UL)_(RB)*N^(RB) _(sc)−1 in the frequency domain, and l is an index rangingfrom 0 to N^(DL/UL) _(symb)−1−1 in the time domain.

Meanwhile, one RB is mapped to one physical resource block (PRB) and onevirtual resource block (VRB). A PRB is defined as N^(DL) _(symb) (e.g.7) consecutive OFDM or SC-FDM symbols in the time domain and N^(RB)_(sc) (e.g. 12) consecutive subcarriers in the frequency domain.Accordingly, one PRB is configured with N^(DL/UL) _(symb)*N^(RB) _(sc)REs. In one subframe, two RBs each located in two slots of the subframewhile occupying the same N^(RB) _(sc) consecutive subcarriers arereferred to as a physical resource block (PRB) pair. Two RBs configuringa PRB pair have the same PRB number (or the same PRB index).

FIG. 3 illustrates the structure of a DL subframe used in a wirelesscommunication system.

A DL subframe is divided into a control region and a data region in thetime domain. Referring to FIG. 3, a maximum of 3 (or 4) OFDM symbolslocated in a front part of a first slot of a subframe corresponds to thecontrol region. Hereinafter, a resource region for PDCCH transmission ina DL subframe is referred to as a PDCCH region. OFDM symbols other thanthe OFDM symbol(s) used in the control region correspond to the dataregion to which a physical downlink shared channel (PDSCH) is allocated.Hereinafter, a resource region available for PDSCH transmission in theDL subframe is referred to as a PDSCH region. Examples of a DL controlchannel used in 3GPP LTE include a physical control format indicatorchannel (PCFICH), a physical downlink control channel (PDCCH), aphysical hybrid ARQ indicator channel (PHICH), etc. The PCFICH istransmitted in the first OFDM symbol of a subframe and carriesinformation about the number of OFDM symbols available for transmissionof a control channel within a subframe. The PHICH carries a HARQ (HybridAutomatic Repeat Request) ACK/NACK(acknowledgment/negative-acknowledgment) signal as a response to ULtransmission.

The control information transmitted through the PDCCH will be referredto as downlink control information (DCI). The DCI includes resourceallocation information for a UE or UE group and other controlinformation. Transmit format and resource allocation information of adownlink shared channel (DL-SCH) are referred to as DL schedulinginformation or DL grant. Transmit format and resource allocationinformation of an uplink shared channel (UL-SCH) are referred to as ULscheduling information or UL grant. The size and usage of the DCIcarried by one PDCCH are varied depending on DCI formats. The size ofthe DCI may be varied depending on a coding rate. In the current 3GPPLTE system, various formats are defined, wherein formats 0 and 4 aredefined for a UL, and formats 1, 1A, 1B, 1C, 1D, 2, 2A, 2B, 3 and 3A aredefined for a DL. Combination selected from control information such asa hopping flag, RB allocation, modulation coding scheme (MCS),redundancy version (RV), new data indicator (NDI), transmit powercontrol (TPC), cyclic shift, cyclic shift demodulation reference signal(DM RS), UL index, channel quality information (CQI) request, DLassignment index, HARQ process number, transmitted precoding matrixindicator (TPMI), precoding matrix indicator (PMI) information istransmitted to the UE as the DCI. Table 3 illustrates an example of theDCI format.

TABLE 3 DCI format Description 0 Resource grants for the PUSCHtransmissions (uplink) 1 Resource assignments for single codeword PDSCHtransmissions 1A Compact signaling of resource assignments for singlecodeword PDSCH 1B Compact signaling of resource assignments for singlecodeword PDSCH 1C Very compact resource assignments for PDSCH (e.g.paging/ broadcast system information) 1D Compact resource assignmentsfor PDSCH using multi-user MIMO 2 Resource assignments for PDSCH forclosed-loop MIMO operation 2A Resource assignments for PDSCH foropen-loop MIMO operation 2B Resource assignments for PDSCH using up to 2antenna ports with UE-specific reference signals 2C Resource assignmentfor PDSCH using up to 8 antenna ports with UE-specific reference signals3/3A Power control commands for PUCCH and PUSCH with 2-bit/ 1-bit poweradjustments 4 Scheduling of PUSCH in one UL Component Carrier withmulti- antenna port transmission mode

In Table 3, formats 0 and 4 are DCI formats defined for UL and formats1, 1A, 1B, 1C, ID, 2, 2A, 2B, 2C, 3, and 3A are DCI formats defined forDL. Various DCI formats other than the formats shown in Table 3 may bedefined.

A plurality of PDCCHs may be transmitted within a control region. A UEmay monitor the plurality of PDCCHs. An eNB determines a DCI formatdepending on the DCI to be transmitted to the UE, and attaches cyclicredundancy check (CRC) to the DCI. The CRC is masked (or scrambled) withan identifier (for example, a radio network temporary identifier (RNTI))depending on usage of the PDCCH or owner of the PDCCH. For example, ifthe PDCCH is for a specific UE, the CRC may be masked with an identifier(for example, cell-RNTI (C-RNTI)) of the corresponding UE. If the PDCCHis for a paging message, the CRC may be masked with a paging identifier(for example, paging-RNTI (P-RNTI)). If the PDCCH is for systeminformation (in more detail, system information block (SIB)), the CRCmay be masked with system information RNTI (SI-RNTI). If the PDCCH isfor a random access response, the CRC may be masked with a random accessRNTI (RA-RNTI). For example, CRC masking (or scrambling) includes XORoperation of CRC and RNTI at the bit level.

The PDCCH is transmitted on an aggregation of one or a plurality ofcontinuous control channel elements (CCEs). The CCE is a logicallocation unit used to provide a coding rate based on the status of aradio channel to the PDCCH. The CCE corresponds to a plurality ofresource element groups (REGs). For example, one CCE corresponds to nineresource element groups (REGs), and one REG corresponds to four REs.Four QPSK symbols are mapped to each REG. A resource element (RE)occupied by the reference signal (RS) is not included in the REG.Accordingly, the number of REGs within given OFDM symbols is varieddepending on the presence of the RS. The REGs are also used for otherdownlink control channels (that is, PDFICH and PHICH). For example, thePCFICH and PHICH include 4 REGs and 3 REGs, respectively. Assuming thatthe number of REGs not allocated to the PCFICH or the PHICH is N_(REG),the number of available CCEs in a DL subframe for PDCCH(s) in a systemis numbered from 0 to N_(CCE)−1, where N_(CCE)=floor(N_(REG)/9).

The number of DCI formats and DCI bits is determined in accordance withthe number of CCEs. The CCEs are numbered and consecutively used. Tosimplify the decoding process, a PDCCH having a format including n CCEsmay be initiated only on CCEs assigned numbers corresponding tomultiples of n. For example, a PDCCH including n consecutive CCEs ma beinitiated only on CCEs satisfying ‘i mod n=0’. Herein, i denotes a CCEindex (or a CCE number).

The number of CCEs used for transmission of a specific PDCCH isdetermined by the eNB in accordance with channel status. For example,one CCE may be required for a PDCCH for a UE (for example, adjacent toeNB) having a good downlink channel. However, in case of a PDCCH for aUE (for example, located near the cell edge) having a poor channel,eight CCEs may be required to obtain sufficient robustness.Additionally, a power level of the PDCCH may be adjusted to correspondto a channel status.

In a 3GPP LTE/LTE-A system, a set of CCEs on which a PDCCH can belocated for each UE is defined. A CCE set in which the UE can detect aPDCCH thereof is referred to as a PDCCH search space or simply as asearch space (SS). An individual resource on which the PDCCH can betransmitted in the SS is called a PDCCH candidate. A set of PDCCHcandidates that the UE is to monitor is defined as the SS. SSs forrespective PDCCH formats may have different sizes and a dedicated SS anda common SS are defined. The dedicated SS is a UE-specific SS (USS) andis configured for each individual UE. The common SS (CSS) is configuredfor a plurality of UEs.

The eNB transmits an actual PDCCH (DCI) on a PDCCH candidate in a searchspace and the UE monitors the search space to detect the PDCCH (DCI).Here, monitoring implies attempting to decode each PDCCH in thecorresponding SS according to all monitored DCI formats. The UE maydetect a PDCCH thereof by monitoring a plurality of PDCCHs. Basically,the UE does not know the location at which a PDCCH thereof istransmitted. Therefore, the UE attempts to decode all PDCCHs of thecorresponding DCI format for each subframe until a PDCCH having an IDthereof is detected and this process is referred to as blind detection(or blind decoding (BD)).

For example, it is assumed that a specific PDCCH is CRC-masked with aradio network temporary identity (RNTI) ‘A’ and information about datatransmitted using a radio resource ‘B’ (e.g. frequency location) andusing transport format information ‘C’ (e.g. transmission block size,modulation scheme, coding information, etc.) is transmitted in aspecific DL subframe. Then, the UE monitors the PDCCH using RNTIinformation thereof. The UE having the RNTI ‘A’ receives the PDCCH andreceives the PDSCH indicated by ‘B’ and ‘C’ through information of thereceived PDCCH.

FIG. 4 illustrates the structure of a UL subframe used in a wirelesscommunication system.

Referring to FIG. 4, a UL subframe may be divided into a data region anda control region in the frequency domain. One or several PUCCHs may beallocated to the control region to deliver UCI. One or several PUSCHsmay be allocated to the data region of the UE subframe to carry userdata.

In the UL subframe, subcarriers distant from a direct current (DC)subcarrier are used as the control region. In other words, subcarrierslocated at both ends of a UL transmission BW are allocated to transmitUCI. A DC subcarrier is a component unused for signal transmission andis mapped to a carrier frequency f₀ in a frequency up-conversionprocess. A PUCCH for one UE is allocated to an RB pair belonging toresources operating on one carrier frequency and RBs belonging to the RBpair occupy different subcarriers in two slots. The PUCCH allocated inthis way is expressed by frequency hopping of the RB pair allocated tothe PUCCH over a slot boundary. If frequency hopping is not applied, theRB pair occupies the same subcarriers.

The PUCCH may be used to transmit the following control information.

-   -   Scheduling request (SR): SR is information used to request a        UL-SCH resource and is transmitted using an on-off keying (OOK)        scheme.    -   HARQ-ACK: HARQ-ACK is a response to a PDCCH and/or a response to        a DL data packet (e.g. a codeword) on a PDSCH. HARQ-ACK        indicates whether the PDCCH or PDSCH has been successfully        received. 1-bit HARQ-ACK is transmitted in response to a single        DL codeword and 2-bit HARQ-ACK is transmitted in response to two        DL codewords. A HARQ-ACK response includes a positive ACK        (simply, ACK), negative ACK (NACK), discontinuous transmission        (DTX), or NACK/DRX. HARQ-ACK is used interchangeably with HARQ        ACK/NACK and ACK/NACK.    -   Channel state information (CSI): CSI is feedback information for        a DL channel. CSI may include channel quality information (CQI),        a precoding matrix indicator (PMI), a precoding type indicator,        and/or a rank indicator (RI). In the CSI, MIMO-related feedback        information includes the RI and the PMI. The RI indicates the        number of streams or the number of layers that the UE can        receive through the same time-frequency resource. The PMI is a        value reflecting a space characteristic of a channel, indicating        an index of a preferred precoding matrix for DL signal        transmission based on a metric such as an SINR. The CQI is a        value of channel strength, indicating a received SINR that can        be obtained by the UE generally when the eNB uses the PMI.

If a UE uses a single carrier frequency division multiple access(SC-FDMA) scheme in UL transmission, a PUCCH and a PUSCH cannot besimultaneously transmitted on one carrier in a 3GPP LTE release-8 orrelease-9 system in order to maintain a single carrier property. In a3GPP LTE release-10 system, support/non-support of simultaneoustransmission of the PUCCH and the PUSCH may be indicated by higherlayers.

The present invention may be applied not only to single-carriercommunication but also to multi-carrier communication.

FIG. 5 is a diagram for explaining single-carrier communication andmulti-carrier communication. Specially, FIG. 5(a) illustrates a subframestructure of a single carrier and FIG. 5(b) illustrates a subframestructure of multiple carriers.

A general wireless communication system transmits/receives data throughone downlink (DL) band and through one uplink (UL) band corresponding tothe DL band (in the case of frequency division duplex (FDD) mode), ordivides a prescribed radio frame into a UL time unit and a DL time unitin the time domain and transmits/receives data through the UL/DL timeunit (in the case of time division duplex (TDD) mode). Recently, to usea wider frequency band in recent wireless communication systems,introduction of carrier aggregation (or BW aggregation) technology thatuses a wider UL/DL BW by aggregating a plurality of UL/DL frequencyblocks has been discussed. A carrier aggregation (CA) is different froman orthogonal frequency division multiplexing (OFDM) system in that DLor UL communication is performed using a plurality of carrierfrequencies, whereas the OFDM system carries a base frequency banddivided into a plurality of orthogonal subcarriers on a single carrierfrequency to perform DL or UL communication. Hereinbelow, each ofcarriers aggregated by carrier aggregation will be referred to as acomponent carrier (CC). For example, three 20 MHz CCs in each of UL andDL are aggregated to support a BW of 60 MHz. The CCs may be contiguousor non-contiguous in the frequency domain. Although a BW of UL CC and aBW of DL CC are the same and are symmetrical, a BW of each componentcarrier may be defined independently. In addition, asymmetric carrieraggregation where the number of UL CCs is different from the number ofDL CCs may be configured. A DL/UL CC for a specific UE may be referredto as a serving UL/DL CC configured at the specific UE.

In the meantime, the 3GPP LTE-A system uses a concept of cell to manageradio resources. The cell is defined by combination of downlinkresources and uplink resources, that is, combination of DL CC and UL CC.The cell may be configured by downlink resources only, or may beconfigured by downlink resources and uplink resources. If carrieraggregation is supported, linkage between a carrier frequency of thedownlink resources (or DL CC) and a carrier frequency of the uplinkresources (or UL CC) may be indicated by system information. Forexample, combination of the DL resources and the UL resources may beindicated by linkage of system information block type 2 (SIB2). In thiscase, the carrier frequency means a center frequency of each cell or CC.A cell operating on a primary frequency may be referred to as a primarycell (Pcell) or PCC, and a cell operating on a secondary frequency maybe referred to as a secondary cell (Scell) or SCC. The carriercorresponding to the Pcell on downlink will be referred to as a downlinkprimary CC (DL PCC), and the carrier corresponding to the Pcell onuplink will be referred to as an uplink primary CC (UL PCC). A Scellmeans a cell that may be configured after completion of radio resourcecontrol (RRC) connection establishment and used to provide additionalradio resources. The Scell may form a set of serving cells for the UEtogether with the Pcell in accordance with capabilities of the UE. Thecarrier corresponding to the Scell on the downlink will be referred toas downlink secondary CC (DL SCC), and the carrier corresponding to theScell on the uplink will be referred to as uplink secondary CC (UL SCC).Although the UE is in RRC-CONNECTED state, if it is not configured bycarrier aggregation or does not support carrier aggregation, a singleserving cell configured by the Pcell only exists.

The eNB may activate all or some of the serving cells configured in theUE or deactivate some of the serving cells for communication with theUE. The eNB may change the activated/deactivated cell, and may changethe number of cells which is/are activated or deactivated. If the eNBallocates available cells to the UE cell-specifically orUE-specifically, at least one of the allocated cells is not deactivatedunless cell allocation to the UE is fully reconfigured or unless the UEperforms handover. Such a cell which is not deactivated unless CCallocation to the UE is full reconfigured will be referred to as Pcell,and a cell which may be activated/deactivated freely by the eNB will bereferred to as Scell. The Pcell and the Scell may be identified fromeach other on the basis of the control information. For example,specific control information may be set to be transmitted and receivedthrough a specific cell only. This specific cell may be referred to asthe Pcell, and the other cell(s) may be referred to as Scell(s).

Among the cells of an eNB, a cell in which carrier aggregation has beenperformed for a UE based on the measurement report from another eNB orUE is referred to as configured cell or serving cell. The serving cellis configured for each UE.

The configured cell for the UE may be a serving cell in terms of the UE.The configured cell for the UE, i.e. the serving cell, prereservesresources for ACK/NACK transmission for PDSCH transmission. An activatedcell refers to a cell configured to be actually used for PDSCH/PUSCHtransmission among configured cells for the UE, and CSI reporting andSRS transmission for PDSCH/PUSCH transmission are performed on theactivated cell. A deactivated cell refers to a cell configured not to beused for PDSCH/PUSCH transmission by the command of an eNB or theoperation of a timer and CSI reporting and SRS transmission are stoppedon the deactivated cell. In order to distinguish between servingcell(s), serving cell indexes may be used. For example, any one ofintegers from 0 to ‘maximum number of carrier frequencies which can beconfigured for the UE at a time minus 1’ may be allocated to one servingcell as the serving cell index. That is, the serving cell index may be alogical index used to identify a specific serving cell among cellsallocated to the UE rather than a physical index used to identify aspecific carrier frequency among all carrier frequencies

As described above, the term cell used in CA is distinguished from theterm cell referring to a prescribed geographic region to which acommunication service is provided by one eNB or one antenna group. Todistinguish between a cell indicating a prescribed geographic region anda cell of CA, in the present invention, the cell of CA is referred to asa CC and the cell of a geographic region is referred to as a cell.

In a CA situation, a plurality of serving CCs may be configured for oneUE. A scheme performed by a control channel for scheduling a datachannel can be divided into existing linked carrier scheduling and crosscarrier scheduling. In link carrier scheduling, a control channeltransmitted on a specific CC schedules only a data channel which is tobe transmitted or received on the specific CC. In contrast, in crosscarrier scheduling, a serving CC having a good channel state may be usedto transmit a UL/DL grant for another serving CC. In cross carrierscheduling, a CC on which a UL/DL grant which is scheduling informationis transmitted may be different from a CC on which UL/DL transmissioncorresponding to the UL/DL grant is performed. In cross carrierscheduling, a control channel schedules, using a carrier indicator field(CIF) in DCI, a data channel transmitted on a CC different from a CC onwhich a PDCCH carrying the DCI is configured.

For reference, in the CA situation, the CIF, which is a field includedin the DCI, is used to indicate a cell for which the DCI carriesscheduling information. The eNB may inform, through a higher layersignal, the UE of whether or not the DCI which the UE will receiveincludes the CIF. That is, the CIF may be configured for the UE by ahigher layer.

When cross carrier scheduling (also referred to as cross-CC scheduling)is applied, a PDCCH for DL assignment may be transmitted on, forexample, DL CC#0 and a PDSCH corresponding to the PDCCH may betransmitted on, for example, DL CC#2. Whether a CIF is present in thePDCCH may be configured semi-statically and UE-specifically (or UEgroup-specifically) by higher layer signaling (e.g. RRC signaling).

The present invention may be applied not only to the PDCCH, the PUCCH,and the PDSCH and/or PUSCH scheduled by the PDCCH but also to an EPDCCH,a PUSCH, and a PDSCH and/or PUSCH scheduled by the EPDCCH.

FIG. 6 illustrates a physical downlink control channel (PDCCH) or anenhanced PDCCH (EPDCCH), and a data channel scheduled by PDCCH/EPDCCH.Particularly, FIG. 6 illustrates the case in which the EPDCCH isconfigured by spanning the fourth symbol to the last symbol of asubframe. The EPDCCH may be configured using consecutive frequencyresources or may be configured using discontinuous frequency resourcesfor frequency diversity.

Referring to FIG. 6, PDCCH 1 and PDCCH 2 may schedule PDSCH 1 and PDSCH2, respectively, and the EPDCCH may schedule another PDSCH. Similarly tothe case of a PDCCH, specific resource assignment units may be definedfor the EPDCCH and the EPDCCH may be configured by a combination of thedefined specific resource assignment units. When the specific resourceassignment units are used, there is an advantage of enabling executionof link adaptation because less resource assignment units can be used toconfigure the EPDCCH in the case of a good channel state and moreresource assignment units can be used to configure the EPDCCH in thecase of a poor channel state. Hereinafter, in order to distinguish abasic unit of the EPDCCH from a CCE which is a basic unit of the PDCCH,the basic unit of the EPDCCH will be referred to as an enhanced CCE(ECCE). It is assumed hereinafter that, for an aggregation level L ofthe EPDCCH, the EPDCCH is transmitted on an aggregation of L ECCEs.Namely, like the aggregation level of the PDCCH, the aggregation levelof the EPDCCH also refers to the number of ECCEs used for transmissionof one DCI. Hereinafter, an aggregation of ECCEs on which the UE iscapable of detecting the EPDCCH thereof will be referred to as an EPDCCHsearch space. DCI carried by the EPDCCH is mapped to a single layer andprecoded.

The ECCEs constituting the EPDCCH may be categorized into a localizedECCE (hereinafter, L-ECCE) and a distributed ECCE (hereinafter, D-ECCE)according to a scheme of mapping the ECCE(s) to RE(s). The L-CCE meansthat REs constituting an ECCE are extracted from the same PRB pair. Ifthe EPDCCH is configured using L-ECCE(s), beamforming optimized for eachUE can be performed. On the other hand, the D-ECCE corresponds to thecase in which REs constituting the ECCE are extracted from different PRBpairs. Unlike the L-ECCE, the D-ECCE can acquire frequency diversity inspite of a restriction on beamforming. In localized mapping, a singleantenna port pε{107,108,109,110} used for EPDCCH transmission is afunction of index(es) of the ECCE for defining the EPDCCH. Indistributed mapping, REs in an EREG are associated with one of twoantenna ports in an alternating manner.

In order for the receiving device 20 to restore a signal transmitted bythe transmitting device 10, an RS for estimating a channel between thereceiving device and the transmitting device is needed. RSs may becategorized into RSs for demodulation and RSs for channel measurement.CRSs defined in the 3GPP LTE system can be used for both demodulationand channel measurement. In a 3GPP LTE-A system, a UE-specific RS(hereinafter, a UE-RS) and a CSI-RS are further defined in addition to aCRS. The UE-RS is used to perform demodulation and the CSI-RS is used toderive CSI. Meanwhile, RSs are divided into a dedicated RS (DRS) and acommon RS (CRS) according to whether a UE recognizes presence thereof.The DRS is known only to a specific UE and the CRS is known to all UEs.Among RSs defined in the 3GPP LTE-A system, the cell-specific RS may beconsidered a sort of the common RS and the DRS may be considered a sortof the UE-RS.

For reference, demodulation may be viewed as a part of the decodingprocess. In the present invention, the terms demodulation and decodingare used interchangeably.

FIG. 7 illustrates configuration of cell specific reference signals(CRSs) and user specific reference signals (UE-RS). In particular, FIG.7 shows REs occupied by the CRS(s) and UE-RS(s) on an RB pair of asubframe having a normal CP.

In an existing 3GPP system, since CRSs are used for both demodulationand measurement, the CRSs are transmitted in all DL subframes in a cellsupporting PDSCH transmission and are transmitted through all antennaports configured at an eNB.

More specifically, CRS sequence r_(l,n) _(s) (m) is mapped tocomplex-valued modulation symbols a_(k,l) ^((p)) used as referencesymbols for antenna port p in slot n_(s) according to the followingequation.a _(k,l) ^((p)) =r _(l,n) _(s) (m′)  [Equation 1]

where n_(s) is the slot number in a radio frame, and l is the OFDMsymbol number within the slot, which is determined according to thefollowing equation.

$\begin{matrix}{{k = {{6m} + {\left( {v + v_{shift}} \right){mod}\; 6}}}{l = \left\{ {{{\begin{matrix}{0,{N_{symb}^{DL} - 3}} & {{{if}\mspace{14mu} p} \in \left\{ {0,1} \right\}} \\1 & {{{if}\mspace{14mu} p} \in \left\{ {2,3} \right\}}\end{matrix}m} = 0},1,\ldots\mspace{14mu},{{{2 \cdot N_{RB}^{DL}} - {1m^{\prime}}} = {m + N_{RB}^{\max,{DL}} - N_{RB}^{DL}}}} \right.}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

where k denotes a subcarrier index, l denotes an OFDM symbol index, andN^(max,DL) _(RB) denotes the largest DL bandwidth configuration,expressed as an integer multiple of N^(RB) _(sc).

Parameters ν and ν_(shift) define locations for different RSs in thefrequency domain and ν is given as follow.

$\begin{matrix}{v = \left\{ \begin{matrix}0 & {{{if}\mspace{14mu} p} = {{0\mspace{14mu}{and}\mspace{14mu} l} = 0}} \\3 & {{{if}\mspace{14mu} p} = {{0\mspace{14mu}{and}\mspace{14mu} l} \neq 0}} \\3 & {{{if}\mspace{14mu} p} = {{1\mspace{14mu}{and}\mspace{14mu} l} = 0}} \\0 & {{{if}\mspace{14mu} p} = {{1\mspace{14mu}{and}\mspace{14mu} l} \neq 0}} \\{3\left( {n_{s}\mspace{14mu}{mod}\; 2} \right)} & {{{if}\mspace{14mu} p} = 2} \\{3 + {3\left( {n_{s}\mspace{14mu}{mod}\; 2} \right)}} & {{{if}\mspace{14mu} p} = 3}\end{matrix} \right.} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack\end{matrix}$

The cell-specific frequency shift ν_(shift) is given by a physical layercell identity N^(cell) _(ID) as follows.ν_(shift) =N _(ID) ^(cell) mod 6  [Equation 4]

A UE may measure CSI using the CRSs and demodulate a signal received ona PDSCH in a subframe including the CRSs. That is, the eNB transmits theCRSs at predetermined locations in each RB of all RBs and the UEperforms channel estimation based on the CRSs and detects the PDSCH. Forexample, the UE may measure a signal received on a CRS RE and detect aPDSCH signal from an RE to which the PDSCH is mapped using the measuredsignal and using the ratio of reception energy per CRS RE to receptionenergy per PDSCH mapped RE. However, when the PDSCH is transmitted basedon the CRSs, since the eNB should transmit the CRSs in all RBs,unnecessary RS overhead occurs. To solve such a problem, in a 3GPP LTE-Asystem, a UE-specific RS (hereinafter, UE-RS) and a CSI-RS are furtherdefined in addition to a CRS. The UE-RS is used for demodulation and theCSI-RS is used to derive CSI. The UE-RS is one type of a DRS. Since theUE-RS and the CRS are used for demodulation, the UE-RS and the CRS maybe regarded as demodulation RSs in terms of usage. Since the CSI-RS andthe CRS are used for channel measurement or channel estimation, theCSI-RS and the CRS may be regarded as measurement RSs.

UE-RSs are transmitted on antenna port(s) p=5, p=7, p=8 or p=7, 8, . . ., u+6 for PDSCH transmission, where a is the number of layers used forthe PDSCH transmission. UE-RSs are present and are a valid reference forPDSCH demodulation only if the PDSCH transmission is associated with thecorresponding antenna port. UE-RSs are transmitted only on RBs to whichthe corresponding PDSCH is mapped. That is, the UE-RSs are configured tobe transmitted only on RB(s) to which a PDSCH is mapped in a subframe inwhich the PDSCH is scheduled unlike CRSs configured to be transmitted inevery subframe irrespective of whether the PDSCH is present.Accordingly, overhead of the RS may be lowered compared to that of theCRS.

In the 3GPP LTE-A system, the UE-RSs are defined in a PRB pair.Referring to FIG. 7, in a PRB having frequency-domain index n_(PRB)assigned for PDSCH transmission with respect to p=7, p=8, or p=7, 8, . .. , υ+6, a part of UE-RS sequence r(m) is mapped to complex-valuedmodulation symbols a_(k,l) ^((p)) in a subframe according to thefollowing equation.a _(k,l) ^((p)) =w _(p)(l′)·r(3·l′·N _(RB) ^(max,DL)+3·n _(PRB)+m′)  [Equation 4]

where w_(p)(i), l′, m′ are given as follows.

$\begin{matrix}{\mspace{79mu}{{w_{p}(i)} = \left\{ {{\begin{matrix}{{\overset{\_}{w}}_{p}(i)} & {{\left( {m^{\prime} + n_{PRB}} \right){mod}\; 2} = 0} \\{{\overset{\_}{w}}_{p}\left( {3 - i} \right)} & {{\left( {m^{\prime} + n_{PRB}} \right){mod}\; 2} = 1}\end{matrix}\mspace{79mu} k} = {{{5m^{\prime}} + {N_{sc}^{RB}n_{PRB}} + {k^{\prime}\mspace{79mu} k^{\prime}}} = \left\{ {{\begin{matrix}1 & {p \in \left\{ {7,8,11,13} \right\}} \\0 & {p \in \left\{ {9,10,12,14} \right\}}\end{matrix}l} = \left\{ {{\begin{matrix}{{l^{\prime}{mod}\; 2} + 2} & {\mspace{14mu}\begin{matrix}{{{if}\mspace{14mu}{in}\mspace{14mu} a\mspace{14mu}{special}\mspace{14mu}{subframe}\mspace{14mu}{with}}\mspace{31mu}} \\{\mspace{14mu}\begin{matrix}{{{configuration}\mspace{14mu} 3},4,{{or}\mspace{14mu} 8}} \\\left( {{see}\mspace{14mu}{Table}\mspace{14mu} 2} \right)\end{matrix}}\end{matrix}} \\{{l^{\prime}{mod}\; 2} + 2 + {3\left\lfloor {l^{\prime}\text{/}2} \right\rfloor}} & \begin{matrix}{\mspace{14mu}\begin{matrix}{{if}\mspace{14mu}{in}\mspace{14mu} a\mspace{14mu}{special}\mspace{14mu}{subframe}} \\{{{with}\mspace{14mu}{configuration}\mspace{14mu} 1},}\end{matrix}} \\{2,6,{{or}\mspace{14mu} 7\mspace{14mu}\left( {{see}\mspace{14mu}{Table}\mspace{14mu} 2} \right)}}\end{matrix} \\{{l^{\prime}{mod}\; 2} + 5} & {{if}\mspace{14mu}{not}\mspace{14mu}{in}\mspace{14mu} a\mspace{14mu}{special}\mspace{14mu}{subframe}}\end{matrix}l^{\prime}} = \left\{ {{{\begin{matrix}{0,1,2,3} & {\mspace{14mu}\begin{matrix}{\mspace{14mu}{{{if}\mspace{14mu} n_{s}\mspace{14mu}{mod}\; 2} = {0\mspace{14mu}{and}\mspace{14mu}{in}\mspace{14mu} a\mspace{14mu}{special}}}} \\{{subframe}\mspace{14mu}{with}\mspace{14mu}{configuration}} \\{1,2,6,{{or}\mspace{14mu} 7\mspace{14mu}\left( {{see}\mspace{14mu}{Table}\mspace{14mu} 2} \right)}}\end{matrix}} \\{0,1} & {\mspace{14mu}\begin{matrix}{\mspace{11mu}{{{if}\mspace{14mu} n_{s}\mspace{14mu}{mod}\; 2} = {0\mspace{14mu}{and}\mspace{14mu}{not}\mspace{14mu}{in}\mspace{14mu}{special}}}} \\{{subframe}\mspace{14mu}{with}\mspace{14mu}{configuration}} \\{1,2,6,{{or}\mspace{14mu} 7\mspace{14mu}\left( {{see}\mspace{14mu}{Table}\mspace{14mu} 2} \right)}}\end{matrix}} \\{2,3} & \begin{matrix}{\mspace{14mu}{{{if}\mspace{14mu} n_{s}\mspace{14mu}{mod}\; 2} = {1\mspace{14mu}{and}\mspace{14mu}{not}\mspace{14mu}{in}\mspace{14mu}{special}}}} \\{{subframe}\mspace{14mu}{with}\mspace{14mu}{configuration}} \\{1,2,6,{{or}\mspace{14mu} 7\mspace{20mu}\left( {{see}\mspace{14mu}{Table}\mspace{14mu} 2} \right)}}\end{matrix}\end{matrix}\mspace{79mu} m^{\prime}} = 0},1,2} \right.} \right.} \right.}} \right.}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

where the sequence w _(p)(i) for normal CP is given according to thefollowing equation.

TABLE 4 Antenna port p [w _(p)(0) w _(p)(1) w _(p)(2) w _(p)(3)] 7 [+1+1 +1 +1] 8 [+1 −1 +1 −1] 9 [+1 +1 +1 +1] 10 [+1 −1 +1 −1] 11 [+1 +1 −1−1] 12 [−1 −1 +1 +1] 13 [+1 −1 −1 +1] 14 [−1 +1 +1 −1]

For antenna port pε{7, 8, . . . , υ+6}, the UE-RS sequence r(m) isdefined as follows.

$\begin{matrix}{{{r(m)} = {{\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {2\; m} \right)}}} \right)} + {j\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {{2m} + 1} \right)}}} \right)}}},{m = \left\{ \begin{matrix}{0,1,\ldots,{{12N_{RB}^{\max,{DL}}} - 1}} & {{normal}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}} \\{0,1,{{\ldots\mspace{14mu} 16N_{RB}^{\max,{DL}}} - 1}} & {{extended}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}\end{matrix} \right.}} & \left\lbrack {{Equation}\mspace{14mu} 7} \right\rbrack\end{matrix}$

c(i) is a pseudo-random sequence defined by a length-31 Gold sequence.The output sequence c(n) of length M_(PN), where n=0, 1, . . . ,M_(PN)-1, is defined by the following equation.c(n)=(x ₁(n+N _(C))+x ₂(n+N _(C)))mod 2x ₁(n+31)=(x ₁(n+3)+x ₁(n))mod 2x ₂(n+31)=(x ₂(n+3)+x ₂(n+2)+x ₂(n+1)+x ₂(n))mod 2  [Equation 8]

where N_(C)=1600 and the first m-sequence is initialized with x₁(0)=1,x₁(n)=0, n=1, 2, . . . , 30. The initialization of the second m-sequenceis denoted by c_(int)=Σ_(i=0) ³⁰x₂(i)·2^(i) with the value depending onthe application of the sequence.

In Equation 7, the pseudo-random sequence generator for generating c(i)is initialized with c_(init) at the start of each subframe according tothe following equation.c _(init)=(└n _(s)/2┘+1)·(2n _(ID) ^((n) ^(SCID) ⁾+1)·2¹⁶ +n_(SCID)  [Equation 9]

In Equation 9, n_(ID) ^((n) ^(SCID) ⁾ indicates a physical layer cellidentifier if the value of n^(DMRS,i) _(ID) is not provided by a higherlayer or DCI format 1A, 2B or 2C is used for DCI associated with PDSCHtransmission. Otherwise, it becomes n^(DMRS,i) _(ID).

In Equation 9, n_(SCID) is 0 unless specified otherwise and is given byDCI format 2B or 2C associated with PDSCH transmission with respect toPDSCH transmission on antenna port 7 or 8. DCI format 2B is a DCI formatfor resource assignment for a PDSCH using a maximum of two antenna portshaving UE-RSs. DCI format 2C is a DCI format for resource assignment fora PDSCH using a maximum of 8 antenna ports having UE-RSs.

Unlike the PDCCH transmitted based on the CRS, the EPDCCH is transmittedbased on the demodulation RS (hereinafter, DM-RS). Accordingly, the UEdecodes/demodulates the PDCCH based on the CRS and decodes/demodulatesthe EPDCCH based on the DM-RS. The DM-RS associated with EPDCCH istransmitted on the same antenna port pε{107,108,109,110} as theassociated EPDCCH physical resource, is present for EPDCCH demodulationonly if the EPDCCH transmission is associated with the correspondingantenna port, and is transmitted only on the PRB(s) upon which thecorresponding EPDCCH is mapped.

In case of normal CP, for the antenna port pε{107,108,109,110} in a PRBn_(PRB) assigned for EPDCCH transmission, a part of the DM-RS sequencer(m) can be mapped to complex-modulation symbols a_(k,l) ^((p)) in asubframe according to the following equation.a _(k,l) ^((p)) =w _(p)(l′)·r(3·l′·N _(RB) ^(max,DL)+3·n _(PRB)+m′)  [Equation 10]

where w_(p)(i), l′, m′ can be given by the following equation.

$\begin{matrix}{\mspace{79mu}{{w_{p}(i)} = \left\{ {{\begin{matrix}{{\overset{\_}{w}}_{p}(i)} & {{\left( {m^{\prime} + n_{PRB}} \right){mod}\; 2} = 0} \\{{\overset{\_}{w}}_{p}\left( {3 - i} \right)} & {{\left( {m^{\prime} + n_{PRB}} \right){mod}\; 2} = 1}\end{matrix}\mspace{79mu} k} = {{{5m^{\prime}} + {N_{sc}^{RB}n_{PRB}} + {k^{\prime}\mspace{79mu} k^{\prime}}} = \left\{ {{\begin{matrix}1 & {p \in \left\{ {107,108} \right\}} \\0 & {p \in \left\{ {109,110} \right\}}\end{matrix}l} = \left\{ {{\begin{matrix}{{l^{\prime}{mod}\; 2} + 2} & \begin{matrix}{\mspace{14mu}\begin{matrix}{{if}\mspace{14mu}{in}\mspace{14mu} a\mspace{14mu}{special}\mspace{14mu}{subframe}} \\{{{with}\mspace{14mu}{configuration}\mspace{14mu} 3},}\end{matrix}} \\{4,{{or}\mspace{14mu} 8\mspace{14mu}\left( {{see}\mspace{14mu}{Table}\mspace{14mu} 2} \right)}}\end{matrix} \\{{l^{\prime}{mod}\; 2} + 2 + {3\left\lfloor {l^{\prime}\text{/}2} \right\rfloor}} & \begin{matrix}{\mspace{14mu}\begin{matrix}{{if}\mspace{14mu}{in}\mspace{14mu} a\mspace{14mu}{special}\mspace{14mu}{subframe}} \\{{{with}\mspace{14mu}{configuration}\mspace{14mu} 1},}\end{matrix}} \\{2,6,{{or}\mspace{14mu} 7\mspace{14mu}\left( {{see}\mspace{14mu}{Table}\mspace{14mu} 2} \right)}}\end{matrix} \\{{l^{\prime}{mod}\; 2} + 5} & {{if}\mspace{14mu}{not}\mspace{14mu}{in}\mspace{14mu} a\mspace{14mu}{special}\mspace{14mu}{subframe}}\end{matrix}\mspace{79mu} l^{\prime}} = \left\{ {{{\begin{matrix}{0,1,2,3} & {\mspace{14mu}\begin{matrix}{\mspace{14mu}\begin{matrix}{{{if}\mspace{14mu} n_{s}\mspace{14mu}{mod}\; 2} = {0\mspace{14mu}{and}}} \\{{in}\mspace{14mu} a\mspace{14mu}{special}\mspace{14mu}{subframe}}\end{matrix}} \\\begin{matrix}{{{with}\mspace{14mu}{configuration}\mspace{14mu} 1},2,} \\{6,{{or}\mspace{14mu} 7\mspace{14mu}\left( {{see}\mspace{14mu}{Table}\mspace{14mu} 2} \right)}}\end{matrix}\end{matrix}} \\{0,1} & {\mspace{14mu}\begin{matrix}{\mspace{11mu}\begin{matrix}{{{if}\mspace{14mu} n_{s}\mspace{14mu}{mod}\; 2} = {0\mspace{14mu}{and}\mspace{14mu}{not}}} \\{{in}\mspace{14mu}{special}\mspace{14mu}{subframe}}\end{matrix}\;} \\\begin{matrix}{{{with}\mspace{14mu}{configuration}\mspace{14mu} 1},2,} \\{6,{{or}\mspace{14mu} 7\mspace{14mu}\left( {{see}\mspace{14mu}{Table}\mspace{14mu} 2} \right)}}\end{matrix}\end{matrix}} \\{2,3} & \begin{matrix}{\mspace{14mu}\begin{matrix}{{{if}\mspace{14mu} n_{s}\mspace{14mu}{mod}\; 2} = {1\mspace{14mu}{and}\mspace{14mu}{not}}} \\{{in}\mspace{14mu}{special}\mspace{14mu}{subframe}}\end{matrix}\mspace{14mu}} \\\begin{matrix}{{{with}\mspace{14mu}{configuration}\mspace{14mu} 1},2,} \\{6,{{or}\mspace{14mu} 7\mspace{14mu}\left( {{see}\mspace{14mu}{Table}\mspace{14mu} 2} \right)}}\end{matrix}\end{matrix}\end{matrix}\mspace{85mu} m^{\prime}} = 0},1,2} \right.} \right.} \right.}} \right.}} & \left\lbrack {{Equation}\mspace{14mu} 11} \right\rbrack\end{matrix}$

where the sequence w _(p)(i) is given by the following table.

TABLE 5 Antenna port p [w _(p)(0) w _(p)(1) w _(p)(2) w _(p)(3)] 107 [+1+1 +1 +1] 108 [+1 −1 +1 −1] 109 [+1 +1 +1 +1] 110 [+1 −1 +1 −1]

For example, in FIG. 7, the REs occupied by the UE-RS(s) of the antennaport 7 or 8 may be occupied by the DM-RS(s) of the antenna port 107 or108 on the PRB to which the EPDCCH is mapped, and the REs occupied bythe UE-RS(s) of antenna port 9 or 10 may be occupied by the DM-RS(s) ofthe antenna port 109 or 110 on the PRB to which the EPDCCH is mapped. Inother words, a certain number of REs are used on each RB pair fortransmission of the DM-RS for demodulation of the EPDCCH regardless ofthe UE or cell if the type of EPDCCH and the number of layers are thesame as in the case of the UE-RS for demodulation of the PDSCH.Hereinafter, the PDCCH and the EPDCCH will be simply referred to asPDCCH.

For the antenna port pε{7, 8, . . . , υ+6}, the UE-RS sequence r(m) forthe EPDCCH is defined by Equation 7. The pseudo-random sequence c(i) ofEquation 7 is defined by Equation 8, and the pseudo-random sequencegenerator for generating c(i) is initialized as c_(init) at the start ofeach subframe according to the following equation.c _(init)=(└n _(s)/2┘+1)·(2n _(ID) ^(EPDCCH)+1)·2¹⁶ +n _(SCID)^(EPDCCI−)  [Equation 12]

The EPDCCH DMRS scrambling sequence initialization parameter n^(EPDCCH)_(SCID) is provided by a higher layer signal.

FIG. 8 illustrates channel state information reference signal (CSI-RS)configurations.

The CSI-RS is a DL RS introduced in the 3GPP LTE-A system for thepurpose of channel measurement, not for the purpose of demodulation. Inthe 3GPP LTE-A system, a plurality of CSI-RS configurations is definedfor CSI-RS transmission. In subframes in which CSI-RS transmission isconfigured, CSI-RS sequence r_(l,n) _(s) (m) is mapped to complexmodulation symbols a_(k,l) ^((p)) used as RSs on antenna port paccording to the following equation.a _(k,l) ^((p)) =w _(l″) ·r _(l,n) _(s) (m′)  [Equation 13]

where w_(l″), k, l are given by the following equation.

$\begin{matrix}{k = {k^{\prime} + {12m} + \left\{ {{\begin{matrix}{- 0} & {{{{for}\mspace{14mu} p} \in \left\{ {15,16} \right\}},{{normal}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}} \\{- 6} & {{{{for}\mspace{14mu} p} \in \left\{ {17,18} \right\}},{{normal}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}} \\{- 1} & {{{{for}\mspace{14mu} p} \in \left\{ {19,20} \right\}},{{normal}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}} \\{- 7} & {{{{for}\mspace{14mu} p} \in \left\{ {21,22} \right\}},{{normal}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}} \\{- 0} & {{{{for}\mspace{14mu} p} \in \left\{ {15,16} \right\}},{{extended}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}} \\{- 3} & {{{{for}\mspace{14mu} p} \in \left\{ {17,18} \right\}},{{extended}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}} \\{- 6} & {{{{for}\mspace{14mu} p} \in \left\{ {19,20} \right\}},{{extended}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}} \\{- 9} & {{{{for}\mspace{14mu} p} \in \left\{ {21,22} \right\}},{{extended}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}}\end{matrix}l} = {l^{\prime} + \left\{ {{\begin{matrix}l^{''} & \begin{matrix}{{{CSI}\mspace{14mu}{reference}\mspace{14mu}{signal}\mspace{14mu}{configurations}\mspace{14mu} 0\text{-}19},} \\{{normal}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}\end{matrix} \\{2l^{''}} & \begin{matrix}{{{CSI}\mspace{14mu}{reference}\mspace{14mu}{signal}\mspace{14mu}{configurations}\mspace{14mu} 20\text{-}31},} \\{{normal}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}\end{matrix} \\l^{''} & \begin{matrix}{{{CSI}\mspace{14mu}{reference}\mspace{14mu}{signal}\mspace{14mu}{configurations}\mspace{14mu} 0\text{-}27},} \\{{extended}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}\end{matrix}\end{matrix}\mspace{79mu} w_{l^{''}}} = \left\{ {{{\begin{matrix}1 & {p \in \left\{ {15,17,19,21} \right\}} \\\left( {- 1} \right)^{l^{''}} & {p \in \left\{ {16,18,20,22} \right\}}\end{matrix}\mspace{79mu} l^{''}} = 0},{{1\mspace{79mu} m} = 0},1,\ldots\mspace{14mu},{{N_{RB}^{DL} - {1\mspace{79mu} m^{\prime}}} = {m + \left\lfloor \frac{N_{RB}^{\max,{DL}} - N_{RB}^{DL}}{2} \right\rfloor}}} \right.} \right.}} \right.}} & \left\lbrack {{Equation}\mspace{14mu} 14} \right\rbrack\end{matrix}$

where (k′, l′) and necessary conditions on n_(s) are given by Table 6and Table 7 in a normal CP and an extended CP, respectively. That is,CSI-RS configurations of Table 6 and Table 7 denote locations of REsoccupied by a CSI-RS of each antenna port in an RB pair.

TABLE 6 Number of CSI reference signals configured CSI reference signal1 or 2 4 8 configuration (k′, l′) n_(s) mod 2 (k′, l′) n_(s) mod 2 (k′,l′) n_(s) mod 2 FS1 and FS2 0 (9, 5) 0 (9, 5) 0 (9, 5) 0 1 (11, 2)  1(11, 2)  1 (11, 2)  1 2 (9, 2) 1 (9, 2) 1 (9, 2) 1 3 (7, 2) 1 (7, 2) 1(7, 2) 1 4 (9, 5) 1 (9, 5) 1 (9, 5) 1 5 (8, 5) 0 (8, 5) 0 6 (10, 2)  1(10, 2)  1 7 (8, 2) 1 (8, 2) 1 8 (6, 2) 1 (6, 2) 1 9 (8, 5) 1 (8, 5) 110 (3, 5) 0 11 (2, 5) 0 12 (5, 2) 1 13 (4, 2) 1 14 (3, 2) 1 15 (2, 2) 116 (1, 2) 1 17 (0, 2) 1 18 (3, 5) 1 19 (2, 5) 1 FS2 only 20 (11, 1)  1(11, 1)  1 (11, 1)  1 21 (9, 1) 1 (9, 1) 1 (9, 1) 1 22 (7, 1) 1 (7, 1) 1(7, 1) 1 23 (10, 1)  1 (10, 1)  1 24 (8, 1) 1 (8, 1) 1 25 (6, 1) 1(6, 1) 1 26 (5, 1) 1 27 (4, 1) 1 28 (3, 1) 1 29 (2, 1) 1 30 (1, 1) 1 31(0, 1) 1

TABLE 7 Number of CSI reference signals configured CSI reference signal1 or 2 4 8 configuration (k′, l′) n_(s) mod2 (k′, l′) n_(s) mod2 (k′,l′) n_(s) mod2 FS1 and 0 (11, 4)  0 (11, 4)  0 (11, 4)  0 FS2 1 (9, 4) 0(9, 4) 0 (9, 4) 0 2 (10, 4)  1 (10, 4)  1 (10, 4)  1 3 (9, 4) 1 (9, 4) 1(9, 4) 1 4 (5, 4) 0 (5, 4) 0 5 (3, 4) 0 (3, 4) 0 6 (4, 4) 1 (4, 4) 1 7(3, 4) 1 (3, 4) 1 8 (8, 4) 0 9 (6, 4) 0 10 (2, 4) 0 11 (0, 4) 0 12 (7,4) 1 13 (6, 4) 1 14 (1, 4) 1 15 (0, 4) 1 FS2 16 (11, 1)  1 (11, 1)  1(11, 1)  1 only 17 (10, 1)  1 (10, 1)  1 (10, 1)  1 18 (9, 1) 1 (9, 1) 1(9, 1) 1 19 (5, 1) 1 (5, 1) 1 20 (4, 1) 1 (4, 1) 1 21 (3, 1) 1 (3, 1) 122 (8, 1) 1 23 (7, 1) 1 24 (6, 1) 1 25 (2, 1) 1 26 (1, 1) 1 27 (0, 1) 1

FIG. 8(a) illustrates 20 CSI-RS configurations 0 to 19 available forCSI-RS transmission through two CSI-RS ports among the CSI-RSconfigurations of Table 6, FIG. 8(b) illustrates 10 available CSI-RSconfigurations 0 to 9 through four CSI-RS ports among the CSI-RSconfigurations of Table 6, and FIG. 8(c) illustrates 5 available CSI-RSconfigurations 0 to 4 through 8 CSI-RS ports among the CSI-RSconfigurations of Table 6. The CSI-RS ports refer to antenna portsconfigured for CSI-RS transmission. For example, referring to Equation14, antenna ports 15 to 22 correspond to the CSI-RS ports. Since CSI-RSconfiguration differs according to the number of CSI-RS ports, if thenumbers of antenna ports configured for CSI-RS transmission differ, thesame CSI-RS configuration number may correspond to different CSI-RSconfigurations.

Unlike a CRS configured to be transmitted in every subframe, a CSI-RS isconfigured to be transmitted at a prescribed period corresponding to aplurality of subframes. Accordingly, CSI-RS configurations vary not onlywith the locations of REs occupied by CSI-RSs in an RB pair according toTable 6 or Table 7 but also with subframes in which CSI-RSs areconfigured. That is, if subframes for CSI-RS transmission differ evenwhen CSI-RS configuration numbers are the same in Table 6 or Table 7,CSI-RS configurations also differ. For example, if CSI-RS transmissionperiods (T_(CSI-RS)) differ or if start subframes (Δ_(CSI-RS)) in whichCSI-RS transmission is configured in one radio frame differ, this may beconsidered as different CSI-RS configurations. Hereinafter, in order todistinguish between a CSI-RS configuration to which a CSI-RSconfiguration number of Table 6 or Table 7 is assigned and a CSI-RSconfiguration varying according to a CSI-RS configuration number ofTable 6 or Table 7, the number of CSI-RS ports, and/or a CSI-RSconfigured subframe, the CSI-RS configuration of the latter will bereferred to as a CSI-RS resource configuration. The CSI-RS configurationof the former will be referred to as a CSI-RS configuration or CSI-RSpattern.

Upon informing a UE of the CSI-RS resource configuration, an eNB mayinform the UE of information about the number of antenna ports used fortransmission of CSI-RSs, a CSI-RS pattern, CSI-RS subframe configurationI_(CSI-RS), UE assumption on reference PDSCH transmitted power for CSIfeedback P_(c), a zero-power CSI-RS configuration list, a zero-powerCSI-RS subframe configuration, etc. CSI-RS subframe configurationI_(CSI-RS) is information for specifying subframe configurationperiodicity T_(CSI-RS) and subframe offset Δ_(CSI-RS) regardingoccurrence of the CSI-RSs. The following table shows CSI-RS subframeconfiguration I_(CSI-RS) according to T_(CSI-RS) and Δ_(CSI-RS).

TABLE 8 CSI-RS- CSI-RS CSI-RS subframe SubframeConfig periodicityT_(CSI-RS) offset Δ_(CSI-RS) I_(CSI-RS) (subframes) (subframes) 0-4 5I_(CSI-RS)  5-14 10 I_(CSI-RS) − 5  15-34 20 I_(CSI-RS) − 15 35-74 40I_(CSI-RS) − 35  75-154 80 I_(CSI-RS) − 75

Subframes satisfying the following equation are subframes includingCSI-RSs.(10n _(f) +└n _(s)/2┘−Δ_(CSI-RS))mod T _(CSI-RS)=0  [Equation 15]

A UE configured in the transmission modes defined after introduction ofthe 3GPP LTE-A system (e.g. transmission mode 9 or other newly definedtransmission modes) may perform channel measurement using a CSI-RS anddecode a PDSCH using a UE-RS.

In conventional systems subject to communication with one node, theUE-RS, CSI-RS, and CRS are transmitted at the same position, andtherefore the UE does not consider a situation in which delay spread,Doppler spread, frequency shift, average received power, and receivedtiming differ among the UE-RS port, CSI-RS port and CRS port. However,for a communication system to which coordinated Multi-Point (CoMP)communication technology allowing more than one node to simultaneouslyparticipate in communication with the UE is applied, the properties maydiffer among the PDCCH port, PDSCH port, UE-RS port, CSI-RS port and/orCRS port. For this reason, the concept of a “quasi co-located antennaport” is introduced for a mode (hereinafter, CoMP mode) in whichmultiple nodes can participate in communication.

With respect to antenna ports, the term “Quasi co-located (QCL)” or“quasi co-location (QCL)” can be defined as follows: if two antennaports are QCL, the UE may assume that the large-scale properties of asignal received through one of the two antenna ports can be inferredfrom the signal received through the other antenna port. The large-scaleproperties include delay spread, Doppler spread, frequency shift,average received power and/or received timing.

With respect to channels, the term QCL may also be defined as follows:if two antenna ports are QCL, the UE may assume that the large-scaleproperties of a channel for conveying a symbol on one of the two antennaports can be inferred from the large-scale properties of a channel forconveying a symbol on the other antenna port. The large-scale propertiesinclude delay spread, Doppler spread, Doppler shift, average gain and/oraverage delay.

One of the two definitions of QCL given above may be applied to thepresent invention. Alternatively, the definition of QCL may be changedto assume that antenna ports for which QCL assumption is established areco-located. For example, QCL may be defined in a manner that the UEassumes that the antenna ports for which QCL assumption is establishedare antenna ports of the same transmit point.

For non-quasi co-located (NQC) antenna ports, the UE cannot assume thesame large-scale properties between the antenna ports. In this case, atypical UE needs to perform independent processing for each NQC antennawith respect to timing acquisition and tracking, frequency offsetestimation and compensation, and delay estimation and Dopplerestimation.

On the other hand, for antenna ports for which QCL assumption can beestablished, the UE performs the following operations:

Regarding Doppler spread, the UE may apply the results of estimation ofthe power-delay-profile, the delay spread and Doppler spectrum and theDoppler spread for one port to a filter (e.g., a Wiener filter) which isused in performing channel estimation for another port;

Regarding frequency shift and received timing, after performing time andfrequency synchronization for one port, the UE may apply the samesynchronization to demodulation on another port;

Further, regarding average received power, the UE may averagemeasurements of reference signal received power (RSRP) over two or moreantenna ports.

For example, if the UE receives a specific DMRS-based DL-related DCIformat (e.g., DCI format 2C) over a PDCCH/ePDCCH, the UE performs datademodulation after performing channel estimation of the PDSCH through aconfigured DMRS sequence. If the UE can make an assumption that a DMRSport configuration received through the DL scheduling grant and a portfor a specific RS (e.g., a specific CSI-RS, a specific CRS, a DL servingcell CRS of the UE, etc.) are QCL, then the UE may apply the estimate(s)of the large-scale properties estimated through a specific RS port toimplementation of channel estimation through the DMRS port, therebyimproving processing performance of the DMRS-based receiver.

FIG. 9 illustrates the concept of a small cell.

In CA for existing systems, when a plurality of CCs is aggregated andused, data transmission and acquisition of a cell ID, transmission ofsystem information, and transmission of a physical control signal areallowed, and thus there exists a PCC capable of accessing a stand-aloneCC and transmitting/receiving a control signal and data. When an SCCwhich is capable of transmitting/receiving data only when CCs areaggregated with the PCC is configured, it is assumed that UL/DL frametime synchronization with the SCC is consistent with timesynchronization of the PCC on the assumption that that CCs not far apartfrom each other in the frequency domain are aggregated. Further, theexisting LTE/LTE-A system considers only a situation in which theaggregated CCs are used by one node, center frequencies neighbors eachother, and thus the frequency properties are similar to each other.

However, the CCs configured for the UE may be used by multiple nodesspaced more than a certain distance from each other, and the centerfrequencies may be spaced apart from each other by an interval greaterthan a certain level. Accordingly, frequency aggregation ofinter-frequencies having different frequency properties may also beconsidered. When different nodes participate in CA using different CCsor the same CC, namely when different cells participate in CA using thesame CC or different CCs, the aggregated CC(s) may be connected by anideal backhaul or a non-ideal backhaul. The ideal backhaul refers to abackhaul having a very high throughput and a very low delay such as adedicated point-to-point connection by means of an optical fiber or anLOS (line of sight) microwave. On the other hand, the non-ideal backhaulrefers to a typical backhaul such as xDSL (digital subscriber line) andNLOS (non line of sight) microwave which are commercially widely used.With the ideal backhaul, it may be presumed that there is no delay inexchanging information between cells or nodes.

Meanwhile, introduction of a small cell whose size, namely the coverageof the node or CC is smaller than that of the existing cell is underconsideration. An existing cell having a wider coverage than the smallcell is called a macro cell. The small cell provides services incoverage narrower than the service coverage of the existing cell due toproperties thereof including power and frequency. Since the small cell,which uses a node of low power, can be readily disposed at indoor andoutdoor hotspots, it is useful when communication traffic soars. Herein,the node of low power generally refers to a node having transmit powerlower than the transmit powers of a macro node and a typical eNB. Forexample, a pico eNB and a femto eNB may be used as low power nodes. Whena UE with low mobility requires high throughput, efficiency of datatransmission may be increased if the UE uses the small cell.

The small cell may be used as a PCC of a specific UE, or used only asthe SCC. Multiple small cells may be established to form a cluster, ormultiple small cells and a macro cell may be established together. Asmall cell cluster formed by a set of multiple small cells may bepresent within the coverage of the macro cell as shown in FIG. 9(a), ormay be independently present out of the coverage of the macro cell asshown in FIG. 9(b).

The UE positioned within the small cell cluster and receiving a servicefrom a specific small cell may need to change the serving cell fromwhich the UE receives the service to another cell, due to worsenedchannel conditions of the specific small cell or mobility of the UE.

FIG. 10 illustrates the conventional handover process. In particular,FIG. 10 illustrates a handover process performed without the mobilitymanagement entity (MME) and the serving gateway (GW) changed. Fordetails of the handover process, refer to 3GPP TS (TechnicalSpecification) 36.300 and 3GPP TS 36.331. Hereinafter, an eNB/cell whichthe UE has accessed to receive a communication service will be referredto as a source eNB/cell, and a new eNB/cell which the UE needs to accesswill be referred to as a target eNB/cell.

-   -   Step 0: The UE context in the source eNB includes information on        roaming restriction given during connection establishment or        recent TA update.    -   Step 1: The source eNB configures a UE measurement process        according to area restriction information. The measurement        provided by the source eNB may assist in controlling connection        and mobility of the UE.    -   Step 2: The UE is triggered to transmit a measurement report        according to a rule set by system information and the like.    -   Step 3: The source eNB determines whether or not to perform        handover of the UE based on the measurement report and radio        resource management (RRM) information.    -   Step 4: The source eNB transmits information necessary for        handover (HO) to the target eNB via a HO request message. The        information necessary for HO includes a UE X2 signaling context        reference, a UE S1 EPC (Evolved Packet Core) signaling context        reference, a target cell ID, and an RRC context containing an        identifier (e.g., a cell radio network temporary identifier        (C-RNTI)) of the UE within the source eNB.    -   Step 6: The target eNB prepares HO with L1/L2R and transmits a        handover request Ack (ACKNOWLEDGE) message to the source eNB.        The handover request Ack message includes a transparent        container transmitted to the UE as an RRC message for performing        handover. The container includes a new C-RNTI and security        algorithm identifiers of the target eNB for selected security        algorithms. The container may include a dedicated RACH (random        access channel) preamble and further include additional        parameters such as access parameters and SIBs.    -   Step 7: The UE receives an RRCConnectionReconfiguration message        containing necessary parameters. The UE is instructed by the        source eNB to perform handover. The necessary parameters may        include a new C-RNTI, and target eNB security algorithm        identifiers. The parameters may also include a dedicated RACH        preamble and target eNB SIBs, which are optional.    -   Step 8: The source eNB sends a serial number (SN) STATUS        TRANSFER message to the target eNB to deliver UL PDCP (Protocol        Data Convergence Protocol) SN receiver status and DL PDCP SN        transmitter status.    -   Step 9: After receiving an RRCConnectionReconfiguration message        containing MobilityControlInformation, the UE performs        synchronization with the target eNB and accesses the target cell        over the RACH. If the dedicated RACH preamble is indicated in        the MobilityControlInformation, access to the target cell over        the RACH is performed in a contention-free process. Otherwise,        the access is performed in a contention-based process. The UE        derives target eNB-specific keys and configures selected        security algorithms to be used in a target cell.    -   Step 10: The network performs uplink allocation and timing        advance.    -   Step 11: If the UE successfully accesses the target cell, the UE        confirms handover by transmitting an        RRCConnectionReconfigurationComplete message (C-RNTI) and        informs the target eNB of completion of the handover process by        transmitting an UL buffer status report. The target eNB        identifies the received C-RNTI through a handover confirm        message and starts to perform data transmission to the UE.    -   Step 12: The target eNB sends a path switch message to the MME        to signal that the UE has switched the cell to another cell.    -   Step 13: The MME sends a User Plane Update Request message to        the serving GW.    -   Step 14: The serving GW switches the DL data path to the target        side. The serving GW may send one or more “end marker” packets        to the source eNB on the old path, and then release a        user-plane/TNL (Transport Network Layer) resource towards the        source eNB.    -   Step 15: The serving GW sends a User Plane Update Response        message to the MME.    -   Step 16: The MME responds to the path switch message using a        path switch Ack message.    -   Step 17: The target eNB sends a UE Context Release message to        inform the source eNB that handover is successful and to trigger        resource release.    -   Step 18: Upon receiving the UE Context Release message, the        source eNB releases user plane-related resources associated with        the radio resource and the UE context.

As can be seen from FIG. 10, various information/parameters areexchanged between network entities in the handover process. When the UEperforms handover to another small cell or switches between the SCCs ina small cell cluster, handover may frequently occur since the coverageof a small cell is small. Frequent handover may apply large overhead tothe UE and the eNB. The present invention proposes a method for the UEto switch the serving cell thereof more quickly and efficiently in orderto reduce system overhead caused by frequent handover. For example, a UEwhich is using a specific small cell in a small cell cluster as aserving cell may need to switch the serving cell to another small cellin the same cluster. According to an embodiment of the presentinvention, the UE may switch the serving cell more quickly andefficiently than when the conventional handover technique is used. Whenthe Pcell of the UE is switched to another cell in the small cellcluster, the process of initial synchronization may not be performed andthe process in which handover is performed may differ from the currentprocess. In addition, radio resource measurement (RRM) different fromthe current measurement may be performed for the small cell in the samesmall cell cluster.

For reference, the RRM is intended to enable the UE and the network toseamlessly manage mobility without significant user intervention byproviding the UE with mobility experience, to ensure efficient use ofthe radio resources, and to provide a mechanism making the eNB satisfypredefined radio resource-related requirements. Main processes performedby the UE to support seamless mobility include cell search, measurement,handover and cell reselection. The eNB may provide measurementconfigurations applicable to the UE to implement RRM. For example, theeNB may trigger measurement by the UE by transmitting, to the UE,measurement configurations including measurement objects, a reportingconfiguration, a measurement identity, a quantity configuration, and ameasurement gap to ensure RRM. The measurement objects, which areobjects on which the UE needs to perform measurement, may include, forexample, a single E-UTRA carrier frequency for intra-frequency andinter-frequency measurement, a single UTRA frequency for inter-RAT(Radio Access Technology) UTRA measurement, a set of GERAN carrierfrequencies for inter-RAT GERAN measurement, and a set of cell(s) on asingle carrier frequency for inter-RAT CDMA2000 measurement. Theintra-frequency measurement refers to measurement on the DL carrierfrequency(s) of the serving cell(s), the inter-frequency measurementrefers to measurement on frequency(s) other than one of the DL carrierfrequency(s) of the serving cell(s). The reporting configuration refersto a list of reporting configurations. Each reporting configuration isestablished with a reporting criterion representing a criterion fortriggering the UE to send a measurement report and a reporting formatindicating the quantities that the UE needs to include in themeasurement report and relevant information. The measurement identity isa list of measurement identities. Each measurement identity links onemeasurement object to one reporting configuration. By configuring aplurality of measurement identifiers, one or more reportingconfigurations may be linked to the same measurement object, and one ormore measurement objects may be linked to the same reportingconfiguration. The measurement identities are used as reference numbersin a measurement report. The quantity configuration defines measurementquantities and relevant filtering which are used for all eventevaluations and relevant reporting of the type of a correspondingmeasurement. One filter may be configured for each measurement. Themeasurement gap indicates a period which the UE can utilize to performmeasurement as no UL/DL transmission is scheduled. Once the UE receivesthe measurement configurations, the UE performs reference signalreceived power (RSRP) measurement and reference signal received quality(RSRQ) measurement using a CRS on a carrier frequency indicated as ameasurement object. The RSRP measurement provides a cell-specific signalstrength metric. RSRP measurement is generally used to determine anorder of candidate cells (or candidate CCs) according to the signalstrength, or is used as an input for determining handover and cellreselection. An RSRP is a linear average of power contribution of REscarrying CRS within a considered frequency bandwidth and defined for aspecific cell (or specific CC). Similar to RSRP, RSRQ, which is intendedto provide a cell-specific signal quality metric, is mainly used todetermine an order of candidate cells (or candidate CCs) according tosignal quality. The RSRQ may be used as an input for handover and cellreselection when, for example, the RSRP measurement does not providesufficient information for performing reliable mobility determination.The RSRQ is defined as “N*RSRP/RSSI”, wherein N denotes the number ofRBs of the RSSI measurement bandwidth. The received signal strengthindicator (RSSI) is defined as all kinds of power including a totalreceived wideband power from all resources including co-channel servingand non-serving cells observed by the UE, adjacent channel interferenceand thermal noise. Accordingly, the RSRQ may be viewed as indicating aratio of the pure RS power to the total power received by the UE.

Embodiment A Serving Cell Switching Operation within a Small CellCluster

The UE may perform handover of switching the serving cell of the UEbetween small cells present in the small cell cluster. The UE may be inthe RRC_connected state or in the RRC_Idle state depending on whetherthe RRC of the UE is logically connected with the RRC of the E-UTRAN.When a user turns on the UE for the first time, the UE searches for aproper cell first, and then stays in the RRC_Idle state in the cell. TheE-UTRAN cannot check the UE staying in the RRC_Idle state in units ofcell, but a core network (CN) manages the UE in units of a tracking area(TA) which is larger than the cell. The UE in the RRC_Idle state maybroadcast system information and paging information while performingdiscontinuous reception (DRX) configured by a non-access stratum (NAS),and may be assigned an identifier for uniquely identifying the UE in aTA. In addition, the UE in the RRC_Idle state may perform selection andreselection of a public land mobile network (PLMN).

All or some of the small cells in the small cell cluster may beRRC_Connected cells which are RRC-connected with the UE. Alternatively,all or some of the small cells may not be RRC_Connected cells, but maybe cells to which the UE can switch the serving cell without assistancefrom a core. In this embodiment, a small cell which can be a target cellof handover and the current serving cell may be subject to the sameaccess barring and PLMN. In addition, if a closed subscriber group (CSG)is applied, it may be assumed that processes necessary for handover havebeen pre-performed. The CSG is a set of UEs having a connectivity accessto a cell. Each CSG has a unique identification number, which is calledCSG identity (CSG ID). The UE may have a list of CSGs to which the UEbelongs, and this CSG list may be changed according to a request fromthe UE or a command from the network. The eNB may deliver the CSG ID ofa CSG which the eNB supports over system information to allow member UEsof the CSG to access a corresponding cell. When the UE finds a CSG cell,it may identify a CSG which the CSG cell supports by reading the CSG IDincluded in the system information. Once the CSG ID is read, the UEregards the CSG cell as a cell which the UE can access only if the UE isa member of the cell. On the other hand, the cell may be configured inan open access mode allowing any UE to access the cell. If the CSG isapplied to a small cell of the present invention, namely, when the smallcell operates in the CSG mode, it may be assumed thatinformation/processes necessary for handover have beenpre-shared/pre-performed by the small cell and the UE since the smallcell and the UE are already aware of the information of the counterpart.Accordingly, when a cell operates in the CSG mode, the UE may switch theserving cell of the UE according to a process of the present invention,not the existing handover process. The present invention proposesmethods which are needed to allow the UE to readily switch the servingcell between the small cells present in the small cell cluster.

Alternative 1

The present invention proposes two cell sets called a small cell set Aand a small cell set B. For example, the small cell set A may includeall or some of the cells in the small cell cluster, and the small cellset B may include all or some of the cells in the small cell set A. Inother words, In other words, the small cell set A may be a subset of thesmall cell cluster, and the small cell set B may be a subset of thesmall cell set A. The cells of the small cell cluster or the small cellset A may be cells neighboring each other by being connected by, forexample, a backhaul. The cells of the small cell set B may be cells ofthe small cell set A which are very probable to become target cells ofhandover or new Scells since they are more adjacent to the UE than theother cells of the small cell set A or have a good channel condition forthe UE. This is simply an example. Since the small cell cluster, smallcell set A and/or small cell set B are configured by the network, andconfiguration of the small cell cluster, small cell set A and/or smallcell set B depends on implementation of the network, details of theconfiguration method may change depending on how the network isimplemented. The present invention proposes a method of signaltransmission/reception between a UE and an eNB on an assumption that theeNB, CN and/or MME can properly configure the small cell cluster, smallcell set A and/or small cell set B.

A serving cell of the UE may be selected from among the cells belongingto the small cell set B. If a target serving cell is not included in thesmall cell set A or the small cell set B, the UE may access the targetserving cell through an access process.

The following relationship may be established among the small cell setA, small cell set B, and the UE. If the UE uses cell 1 which is a cellbelonging to the small cell set A as a serving cell of the UE, an eNBoperating/controlling cell 1, an eNB operating/controlling the servingcell that was used before the UE performed handover to cell 1, or an eNBoperating/controlling the Pcell (or the macro cell) of the UE mayprovide the UE with parameter sets for all cells included in the smallcell set A through an RRC signal. When content of the parameter sets forthe respective cells is updated, the eNB operating/controlling cell 1which is the current serving cell of the UE or the eNBoperating/controlling the Pcell (or the macro cell) of the UE may informthe UE of the updated parameter sets through a higher layer signal.

Some examples of the parameter set necessary for the present inventionare given below.

-   -   Master information block (MIB)-related parameters    -   Downlink bandwidth    -   PHICH configuration    -   System frame number (SFN): the SFNs of cells may be aligned in        the small cell cluster. In other words, for the cells in the        small cell cluster, the subframes at the same start time may        have the same SFN. In this case, the SFN may be omitted.    -   S1B1-related parameters    -   PLMN identities of the network: it may be assumed that the same        PLMN is given in the small cell cluster. In this case, the PLMN        information may be omitted.    -   Tracking area code (TAC) and cell ID    -   Cell barring status    -   q-RxLevMin indicating a minimum Rx level in a cell for        fulfilling the cell selection criteria    -   Transmission times and periodicities of other SIBs    -   PRACH configuration    -   Cell ON/OFF Related Information    -   ON/OFF state period/duration    -   Cell ID used in the OFF state

If the cells in the small cell set A have the same bandwidth andoperating frequency, the parameter set for each cell included in thesmall cell set A may include parameters related to information on PDSCHmapping for CoMP and information on an indicator of quasi-co-location(QCL) between DMRS and CSI-RS. These parameters may include, forexample, the following parameters.

-   -   Number of CRS ports    -   Frequency shift of CRS ν_(shift)    -   MBSFN (Multimedia Broadcast multicast service Single Frequency        Network) subframe configuration list    -   Configuration of zero power CSI-RS    -   PDSCH start symbol    -   Non-zero power CSI-RS resource index

The number of CRS ports may represent the number of CRS port(s) whichare associated with PDSCH transmission or QCL with PDSCH antennaport(s), the frequency shift of CRS may represent the frequency shiftν_(shift) of CRS port(s) which are QCL with PDSCH antenna port(s). TheMBSFN subframe configuration list may indicate subframes reserved forthe MBSFN on downlink, and the configuration of zero power CSI-RS mayindicate a zero power CSI-RS configuration list and a configuration of azero power CSI-RS subframe. The PDSCH start symbol may indicate thestart OFDM symbol of a PDSCH with respect to a corresponding servingcell, and the non-zero power CSI-RS resource index may indicate a CSI-RSresource which is QCL with the PDSCH antenna port(s). The parametersdescribed above may be configured to determine PDSCH RE mapping andPDSCH antenna port QCL.

The small cell set B may be a subset of the small cell set A. If theserving cell of the UE is cell 1 which is a cell belonging to the smallcell set A, an eNB operating/controlling cell 1, an eNBoperating/controlling the serving cell that was used before the UEperformed handover to cell 1, or an eNB operating/controlling the Pcell(or the macro cell) of the UE may inform the UE of the cells belongingto the small cell set B through an RRC signal or a medium access control(MAC) control element (CE).

The cells included in the small cell set B may be used as cells to whichthe serving cell of the UE can be switched by an RRC configuration, aMAC CE, or a request over a PDCCH which are different from those of theconventional handover technique. In the present invention, cell 1 whichis the current serving cell of the UE belonging to the small cell set Aalso belongs to the small cell set B. The next serving cell for the UEto use may be selected from among the cells included in the small cellset B. If a new small cell set B is configured for the UE, the currentserving cell of the UE should belong to the new small cell set B. Inaddition, if a new small cell set B is configured for the UE, a cellconfiguring the new small cell set B should be included in the new smallcell set B. In the present invention, the small cell set B may beconfigured as one cell. In this case, the cell belonging to the smallcell set B may be the current serving cell of the UE.

To designate one of the cells in the small cell set B as a new servingcell of the UE, an eNB operating/controlling the current serving cell ofthe UE or an eNB operating/controlling the Pcell (or the macro cell) ofthe UE may switch the serving cell of the UE through a new RRCreconfiguration process different from the existing handover process. Inthis case, the serving cell of the UE may be more quickly switched sincethe cells included in the small cell set B are in the RRC_Connectedstate or there is no intervention of the core/MME. Alternatively, theeNB operating/controlling the current serving cell of the UE or the eNBoperating/controlling the Pcell (or the macro cell) of the UE may switchthe serving cell of the UE by transmitting, to the UE, a MAC CE which isset to deactivate the current serving cell among the cells of the smallcell set B and to activate another cell of the small cell set B.Alternatively, the eNB operating/controlling the current serving cell ofthe UE or the eNB operating/controlling the Pcell (or the macro cell) ofthe UE may switch the serving cell of the UE through a PDCCH/ePDCCH. Inthis case, information on whether or not the serving cell of the UE isswitched to another cell and/or information on a new serving cell suchas an index may be included in the PDCCH/ePDCCH. The index of a cell towhich the serving cell is to be switched, namely the target cell may besent to the UE by reusing an existing field (e.g., the TPC field) of theDCI or by adding a new field to the DCI.

Once the UE receives the information such as the index of the newserving cell, the UE may operate, recognizing the corresponding cell asthe serving cell thereof, until it receives a next request for switchingof the serving cell. In addition, the UE may receive a DL signal andtransmit a UL signal, using the parameter set for the new serving cell.In terms of PDSCH reception, if the UE receives information such as theindex of the new serving cell, the UE may receive a PDSCH using theparameter set for the new serving cell. For example, if the UE receivesinformation such as the index of the new serving cell through a PDCCH,the UE may receive a PDSCH using the parameter set for the new servingcell from the subframe in which the UE has received the PDCCH (until theUE receives another serving cell switch request). Alternatively, if theUE receives information such as the index of the serving cell of the UEthrough every PDCCH, the UE may receive the PDSCH of the subframe inwhich the PDCCH has been received, using the parameter set of theserving cell indicated by the PDCCH.

FIG. 11 is a diagram illustrating cell sets according to one embodimentof the present invention, and FIGS. 12 and 13 are diagrams illustratingparameter set(s) according to one embodiment of the present invention.

As shown in FIG. 11, a small cell set A may exist and some of the cellsof the small cell set A may belong to a small cell set B. In this case,the serving cell of the UE may be one of the cells belonging to thesmall cell set B. For example, if the cells in the small cell set A havethe same bandwidth operating frequency, the parameter set for each cellmay include parameters as shown in FIG. 12.

If there are 10 small cells in the small cell set A, the eNB may informthe UE of the parameter set for each of the 10 cells. Table 9exemplarily shows RRC information for configuring the small cell set A,Tables 10 and 11 exemplarily show RRC information on a parameter setcorresponding to the small cell set A and corresponding fielddescriptions.

TABLE 9 SmallCellSetA information element -- ASN1START   ...SmallCellSetA-InfoList ::= SEQUENCE (SIZE (1..maxSmallCell)) OFSmallCellInfo SmallCellInfo::= SEQUENCE { SmallCellIndex SmallCellIndex,cellIdentification SEQUENCE { physCellId PhysCellId, dl-CarrierFreqARFCN-ValueEUTRA } ... } -- ASN1STOP

TABLE 10 SmallCellSetParameter information element -- ASN1STARTSmallCellParameter ::= SEQUENCE { SmallCellIndex SmallCellIndexcrs-PortsCount ENUMERATED {n1, n2, n4, spare1 }, crs-FreqShift INTEGER(0...5), mbsfn-SubframeConfig-r11 MBSFN-SubframeConfig OPTIONAL, -- NeedOR pdsch-Start ENUMERATED {reserved, n1, n2, n3, n4, assigned} }OPTIONAL, -- Need OP csi-RS-ConfigZPId CSI-RS-ConfigZPId,qcl-CSI-RS-ConfigNZPId CSI-RS-ConfigNZPId OPTIONAL, -- Need OR ... } ...} -- ASN1STOP

TABLE 11 SmallCellSetParameter field descriptions antennaPortsCountParameter represents the number of cell specific antenna ports where an1corresponds to 1, an2 to 2 antenna ports etc. MBSFN-SubframeConfigdefines subframes that are reserved for MBSFN in downlink pdsch-StartThe starting OFDM symbol of PDSCH for the concerned serving cell, see3GPP TS 36.213. Values 1, 2, 3 are applicable when dl-Bandwidth for theconcerned serving cell is greater than 10 resource blocks, values 2, 3,4 are applicable when dl-Bandwidth for the concerned serving cell isless than or equal to 10 resource blocks, see 3GPP TS 36.211. Value n1corresponds to 1, value n2 corresponds to 2 and so on. csi-RS-ConfigZPIdidentifies a CSI-RS resource configuration for which UE assumes zerotransmission power, as configured by the IE CSI-RS-ConfigZP. Theidentity is unique within the scope of a carrier frequency.qcl-CSI-RS-ConflgNZPId Indicates the CSI-RS resource that is quasico-located with the PDSCH antenna ports, see 3GPP TS 36.213. E-UTRANconfigures this field if and only if the UE is configured withqcl-Operation set to typeB.

The eNB may further inform the UE of which cells of the 10 cells in thesmall cell set A belong to the small cell set B. The eNB may notify theUE of the cell belonging to the small cell set B using, for example, abitmap. For example, if 10 cells from cell 1 to cell 10 belong to thesmall cell set A, and cell 2, cell 3 and cell 5 belong to the small cellset B, the eNB may inform the UE of the small cell set B configured forthe UE by transmitting a bitmap set to “0110100000” as shown in thefollowing table.

TABLE 12 Cell ID in a Cell Cell Cell Cell Cell Cell Cell Cell Cell Cellsmall cell set 1 2 3 4 5 6 7 8 9 10 A Small cell set Bit 0 Bit 1 Bit 2Bit 3 Bit 4 Bit 5 Bit 6 Bit 7 Bit 8 Bit 9 B indication field Value 0 1 10 1 0 0 0 0 0z

In another example, the small cell set B may be configured for the UEusing an activation/deactivation MAC CE. The maximum number of cellswhich can be included in the small cell set A and the maximum number ofcells which can be included in the small cell set B may be set to valuespre-designated for the system. When it is assumed that the maximumnumber of cells or parameter sets included in the small cell set A is10, an example of the MAC CE for configuring the small cell set B may begiven as shown in the following table.

TABLE 13 C₁₀ C₉ C₈ C₇ C₆ C₅ C₄ C₃ C₂ C₁

In Table 13, C_(i) indicates activation/deactivation of cell i orparameter set i of the small cell set A. A cell or parameter setcorresponding to C_(i) set to 1 may be activated, and a cell orparameter set corresponding to C_(i) set to 0 may be deactivated.

Referring to FIG. 13, when the small cell set B including cell 2, cell 3and cell 5 is configured for the UE, a DL signal may be received and/ora UL signal may be transmitted using one of the parameter sets for cell2, cell 3 and cell 5 among the 10 parameter sets associated with thesmall cell set A. The eNB operating/controlling the current serving cellof the UE or the eNB operating/controlling the Pcell (or the macro cell)of the UE may select a new serving cell of the UE from among the cellsbelonging to the small cell set B and inform the UE of the new servingcell through an RRC signal, a MAC CE, or a PDCCH, thereby informing theUE of a parameter set which the UE can actually use among the parametersets for cell 2, cell 3 and cell 5. That is, one of cell 2, cell 3 andcell 5 may be selected as a new serving cell of the UE. The UE may benotified of the new serving cell by transmitting one of the valuesmatched to the cells of the small cell set B in the manner of one-to-onecorrespondence to the UE.

TABLE 14 Value New serving cell 0 Cell 2 1 Cell 3 2 Cell 5

For example, in Table 14, if the UE receives 0 as the index of the newserving cell, the UE may recognize cell 2 as the new serving cellthereof, and thus receive a signal and/or transmit a UL signal using aparameter set for cell 2. In terms of PDSCH reception, the UE receives aPDSCH using the parameter set for cell 2.

Alternative 2

The small cell set A and the small cell set B may be represented by setsof parameter sets therefor, rather than by sets of cells thereof. Ifcell 1 in the small cell set A is used as the serving cell of the UE, aneNB operating/controlling cell 1, an eNB operating/controlling theserving cell that was used before the UE performed handover to cell 1,or an eNB operating/controlling the Pcell (or the macro cell) of the UEmay provide the UE with the parameter sets included in the small cellset A through an RRC signal. When content of the respective parametersets is updated, the eNB operating/controlling cell 1 which is thecurrent serving cell of the UE or the eNB operating/controlling thePcell (or macro cell) of the UE may transmit, to the UE, the updatedparameter set(s) through a higher layer signal. A parameter set mayinclude cell-specific parameters and/or UE-specific parameters foroperation of the UE. Examples of the cell-specific parameters and theUE-specific parameters are given below.

-   -   Cell-specific parameters    -   cell ID (cell ID)    -   MIB-related parameters    -   SIB1-related parameters    -   PRACH configuration    -   Cell ON/OFF-related information    -   Information on PDSCH mapping for CoMP and information on QCL        between DMRS and CSI-RS (e.g., the number of CRS ports,        configuration of zero power CSI-RS, PDSCH start symbol, and        CSI-RS resource index)    -   UE-specific parameters    -   Information on PDSCH mapping for CoMP and information on QCL        between DMRS and CSI-RS (e.g., the number of CRS ports,        configuration of zero power CSI-RS, PDSCH start symbol, and        CSI-RS resource index)    -   SRS transmission-related parameters

The small cell set B may be configured with all or some of parametersets in the small cell set A. If the serving cell of the UE is cell 1which is a cell belonging to the small cell set A, an eNBoperating/controlling cell 1, an eNB operating/controlling the servingcell that was used before the UE performed handover to cell 1, or an eNBoperating/controlling the Pcell (or the macro cell) of the UE may informthe UE of the parameter sets belonging to the small cell set B throughan RRC signal or an MAC CE. For example, a MAC CE activated by parameterset(s) belonging to the small cell set B among the parameter setsbelonging to the small cell set A may be transmitted to the UE, or anRRC signal containing index(s) of parameter set(s) belonging to thesmall cell set B among the indexes of the parameters belonging to thesmall cell set A may be transmitted to the UE.

The parameter set(s) included in the small cell set B may be configuredas parameter set(s) for the UE through an RRC configuration, a MAC CE,or a PDCCH which are different from those of the conventional handovertechnique. The small cell set B may include only one parameter set. Inthis case, the small cell set B may a parameter set which the UE iscurrently using.

To designate one of the parameter sets in the small cell set B as a newparameter set for the UE to use, an eNB operating/controlling thecurrent serving cell of the UE or an eNB operating/controlling the Pcell(or the macro cell) of the UE may switch the parameter set for the UE touse through a new RRC reconfiguration process. Alternatively, the eNBoperating/controlling the current serving cell of the UE or the eNBoperating/controlling the Pcell (or the macro cell) of the UE may switchparameter sets for the UE to use by transmitting, to the UE, a MAC CEwhich is set to activate a new parameter set for the UE to use among theparameter sets in the small cell set B and deactivate parameter set(s)that are not used among the parameter sets. Alternatively, the eNBoperating/controlling the current serving cell of the UE or the eNBoperating/controlling the Pcell (or the macro cell) of the UE may make arequest for switching of a parameter set used by the UE to anotherparameter set through a PDCCH/ePDCCH. In this case, the PDCCH/ePDCCH maycontain information on whether or not the parameter set used by the UEis switched to another parameter set and/or information on the index ofa parameter set for the new serving cell. The index of a parameter setto which the parameter set is to be switched may be transmitted to theUE by setting the index in an existing field (e.g., a TPC field) of theDCI or by adding a new field to the DCI and setting the index in the newfield. Once the UE receives information such as the index of the newparameter set, the UE may operate, recognizing the new parameter set asa parameter set for the UE to use until the UE receives the next requestfor switch of the parameter set. In terms of PDSCH reception, if the UEreceives information such as the index of the new parameter set, the UEmay receive a PDSCH using the indicated new parameter set. For example,if the UE receives the index of a new parameter set through a PDCCH, theUE may receive a PDSCH using the new parameter set from the subframe inwhich the PDCCH has been detected (until the UE receives the index ofthe next parameter set associated with switching between the parametersets). In another example, if the UE receives information such as theindex of the parameter set of the UE through every PDCCH, the UE mayreceive a PDSCH in the subframe in which the PDCCH has been received,using the parameter set indicated by the PDSCH.

Two or more parameter sets in the small cell set B may be designated asparameter sets for the UE to use. For example, a UE may use twodesignated parameter sets. When the two parameter sets used by one UEare defined as Pset_C and Pset_U, the parameters constituting Pset_Cdiffer from those constituting Pset_U, and the parameter sets does notinclude the same parameter. For example, Pset_C may be a set ofcell-specific parameters, and Pset_U may be a set of UE-specificparameters. When new parameter sets are designated for the UE, only someparameter sets of the two or more parameter sets which the UE is usingmay be switched to the new parameter sets.

Alternatively, two or more small cell sets A may be configured. Forexample, suppose that two small cell sets A configured for the UE aresmall cell set A_C and small cell set A_U. The parameters included inthe parameter sets belonging to small cell set A_C may differ from theparameters included in the parameter sets belonging to small cell setA_U, and no identical parameter can be included in a parameter set insmall cell set A_C and a parameter set in small cell set_U at the sametime. For example, the parameter sets belonging to small cell set A_Cmay be sets of cell-specific parameters, and the parameter setsbelonging to small cell set A_U may be sets of UE-specific parameters.Similar to configuration of the two small cell sets A, there may be twosmall cell sets B in relation to the small cell sets A. For example,small cell set B_C may be configured with all or some of the parametersets in small cell set A_C, and small cell set B_U may be configuredwith all or some of the parameter sets in small cell set A_U. Oneparameter set may be designated in each of the multiple small cell setsB and the UE may use the designated parameter sets. For example, whensmall cell set B_C and small cell set B_U are configured, one parameterset may be designated in each of small cell set B_C and small cell setB_U such that the UE uses the designated parameter sets. In this case,the UE may use a cell-specific parameter set provided from small cellset B_C and a UE-specific parameter set from small cell set B_U. Asanother example, when the UE has two small cell sets A defined as smallcell set A_C and small cell set A_U, small cell set A_C may beconfigured as a set of cells, and small cell set A_U may be configuredas a set of parameter sets. In this case, the UE may be provided withparameter sets for the respective cells belonging to small cell set A_Cas described above in Alternative 1. In addition, as described inAlternative 2, the UE may be provided with the parameter sets includedin small cell set A_U. In this case, each of the parameter sets for thecells belonging to small cell set A_C may include cell-specificparameters for the corresponding cell. In addition, each of theparameter sets belonging to small cell set A_U may be configured as aset of UE-specific parameters. Similar to the small cell sets A, twosmall cell sets B may be provided in relation to the small cell sets A.Small cell set B_C may be a set of all or some of the cells included insmall cell set A_C, and small cell set B_U may be a set of all or someof the parameter sets included in small cell set A_U. In this case, theUE may be informed of a serving cell of the UE designated in small cellset B_C and use a parameter set related to the designated cell. Inaddition, the UE may use a parameter set designated in small cell setB_U. For example, a serving cell for the UE to use may be designated insmall cell set B_C, and the UE may use a cell-specific parameter setrelated to the designated cell. The UE may also be provided with aUE-specific parameter set for the UE to use from small cell set B_U.

Alternative 3

If cell 1 which is a cell belonging to the small cell set A is used asthe serving cell of the UE, an eNB operating/controlling cell 1, an eNBoperating/controlling the serving cell that was used before the UEperformed handover to cell 1, or an eNB operating/controlling the Pcell(or the macro cell) of the UE may provide the UE with (common)information on all cells included in the small cell set A or commonparameter(s) which are needed for the UE to operate in the small cellset A. For example, the following parameter(s) may be included in thecommon parameter(s).

-   -   Bandwidth    -   MBSFN subframe configuration    -   UL/DL configuration    -   Special subframe configuration    -   Operating frequency

The UE may determine that all the cells in small cell set A have thesame value for the provided common parameter(s). Alternatively, the UEmay determine that the UE can operate using the provided commonparameter(s) in the small cell set A.

The small cell set B may be configured with all or some of the cells inthe small cell set A. If cell 1 which is a cell belonging to the smallcell set A is used as the serving cell of the UE, an eNBoperating/controlling cell 1, an eNB operating/controlling the servingcell that was used before the UE performed handover to cell 1, or an eNBoperating/controlling the Pcell (or the macro cell) of the UE maysignal, to the UE, the cells belonging to the small cell set B throughan RRC signal or a MAC CE. Additionally, the eNB operating/controllingcell 1, the eNB operating/controlling the serving cell that was usedbefore the UE performed handover to cell 1, or the eNBoperating/controlling the Pcell (or the macro cell) of the UE may informthe UE of the cell-specific parameters for each of the cells belongingto the small cell set B through the RRC signal or the MAC CE.

The cells included in the small cell set B may be used as cells to whichthe serving cell of the UE can be switched through an RRC configuration,a MAC CE, or a PDCCH which are different from those of the conventionalhandover technique.

Cell 1 which is the current serving cell of the UE belonging to thesmall cell set A is included in the small cell set B. the next servingcell for the UE to use is selected from among the cells included in thesmall cell set B. The serving cell of the UE or a cell configuring thesmall cell set B may always belong to a new small cell set B. If a newsmall cell set B is configured for the UE, the current serving cell ofthe UE should belong to the new small cell set B. In addition, if thenew small cell set B of the UE is configured, a cell configuring the newsmall cell set B should be included in the new small cell set B.

Specifically, the small cell set B may include only one cell. In thiscase, the small cell set B may be the current serving cell of the UE.

To designate one of the cells in the small cell set B as a new servingcell of the UE, an eNB operating/controlling the current serving cell ofthe UE or an eNB operating/controlling the Pcell (or the macro cell) ofthe UE may switch the serving cell of the UE through a new RRCreconfiguration process different from the existing handover process. Inthis case, the serving cell of the UE may be more quickly switched sincethe cells included in the small cell set B are in the RRC_Connectedstate or there is no intervention of the core/MME. Alternatively, theeNB operating/controlling the current serving cell of the UE or the eNBoperating/controlling the Pcell (or the macro cell) of the UE may switchthe serving cell of the UE by transmitting, to the UE, a MAC CE which isset to activate a new cell and deactivate the other cell(s).Alternatively, the eNB operating/controlling the current serving cell ofthe UE or the eNB operating/controlling the Pcell (or the macro cell) ofthe UE may make a request for switching of the serving cell of the UEthrough a PDCCH/ePDCCH. In this case, information on whether or not theserving cell of the UE is switched to another cell and/or informationsuch as the index of a new serving cell may be included in thePDCCH/ePDCCH. The index of a cell to which the serving cell is to beswitched, namely the target cell may be sent to the UE by reusing anexisting field (e.g., the TPC field) of the DCI or by adding a new fieldto the DCI.

Once the UE receives the information such as the index of the newserving cell, the UE may operate, recognizing the corresponding cell asthe serving cell thereof, until it receives a next request for switchingof the serving cell. The UE may also operate using a cell-specificparameter set for the indicated new serving cell in addition to a set ofcommon parameters. In terms of PDSCH reception, if the UE receivesinformation such as the index of the new serving cell, the UE mayreceive a PDSCH using the cell-specific parameter set for the indicatednew serving cell in addition to the set of common parameters. If the UEreceives information such as the index of the new serving cell through aPDCCH, the UE may receive a PDSCH using the cell-specific parameter setfor the new serving cell indicated by the PDCCH in addition to the setof common parameters from the subframe in which the UE has received thePDCCH (until the UE receives a PDCCH carrying information on anothercell-specific parameter set). Alternatively, if the UE receivesinformation such as the index of the serving cell of the UE throughevery PDCCH, the UE may receive the PDSCH in the subframe in which theUE has received a PDCCH using the cell-specific parameter set for thenew serving cell indicated by the PDCCH in addition to the set of commonparameters.

Determination of Switching of the Serving Cell

An eNB operating/controlling the serving cell of the UE or an eNBoperating/controlling the Pcell (or the macro cell) of the UE referencesan RRM value of each cell reported by the UE in order to performdetermination related to switching of the serving cell of the UE. TheeNB may consider switching the serving cell of the UE based on themeasurement values for the other cells in the small cell set B in thefollowing situations.

-   -   The serving cell is worsened below an absolute threshold.        Herein, the absolute threshold may be set separately from the        threshold in existing handover. Specifically, the absolute        threshold may have a value greater than that of the threshold in        existing handover.    -   The serving cell candidate becomes better than an absolute        threshold. Herein, the absolute threshold may be set separately        from the threshold in existing handover. Specifically, the        absolute threshold may have a value less than that of the        threshold in existing handover.    -   A serving cell in the serving cell set B becomes better than the        offset relative to the serving cell. Herein, the offset may be        set separately from the offset in existing handover.        Specifically, the offset may have a value less than that of the        offset in existing handover.    -   The serving cell is worsened below an absolute threshold and a        cell in the serving cell set B becomes better than another        absolute threshold. Herein, the two absolute thresholds may be        set separately from the absolute thresholds in the existing        handover.

Operation of UE in Switching Serving Cell

The time taken for a signal transmitted from a UE to reach an eNB maydepend on the radius of the cell, the position of the UE in the cell,and the speed of movement of the UE. That is, if the eNB does not managetransmission timing for each UE, a transmitted signal from a specific UEpossibly interferes with a transmitted signal from another UE, and theerror rate of the received signal on the eNB increases. Morespecifically, for a UE attempting to transmit a signal at the edge ofthe cell, the time taken for the transmitted signal to reach the eNBwill be longer than the time taken for a signal transmitted from a UE atthe center of the cell to reach the eNB. In other words, the time takenfor a signal transmitted from a UE at the center of the cell to reachthe eNB will be shorter than the time taken for a signal transmittedfrom the UE at the edge of the cell to reach the eNB. To preventinterference between data or signals transmitted from all UEs in thecell, the transmitted data or signals need to be adjusted to be receivedwithin each effective time boundary, and thus the eNB needs to properlyadjust transmission timing of the UE according to the condition of theUE. This adjustment is called timing advance management or timingalignment management. One method to manage the UL time alignment may bea random access procedure. The random access procedure (also called anRACH process) is performed for the following Pcell-related events:initial access in the RRC_Idle state, and RRC connectionre-establishment process; handover, arrival of DL data during theRRC_Connected state requiring the random access procedure, arrival of ULdata during the RRC_Connected state requiring the random accessprocedure, and positioning during the RRC_Connected state requiring therandom access procedure. The random access procedure may be performed onthe Scell to establish a time alignment for a timing advance group (TAG)which uses the same timing advance value. The random access procedure isinitiated by a PDCCH order or a MAC sublayer. Before the random accessprocedure can be initiated, 1) information (prach-ConfigIndex)representing an available set of PRACH resources for transmission of arandom access preamble and 2) information representing the groups ofrandom access preambles and a set of available random access preamblesin each group are assumed.

The random access procedure is divided into a contention-based randomaccess procedure and a non-contention-based random access procedureaccording to how the UE selects an RACH preamble. In thecontention-based random access procedure, the UE randomly selects oneRACH preamble from a set of specific RACH preambles and uses the same.In the non-contention-based random access procedure, the UE uses aspecific RACH preamble allocated thereto by the eNB. Thecontention-based random access procedure may be performed according tothe following steps.

1) Random Access Preamble

In contention-based random access, the UE may randomly select one randomaccess preamble from a set of random access preambles (also called RACHpreambles) indicated through system information or a handover command,and select and transmit a PRACH resource for transmission of the randomaccess preamble.

2) Random Access Response

After the UE transmits the random access preamble, the UE attempts toreceive a random access response thereof indicated by the eNB throughthe system information or handover command within a random accessresponse reception window. More specifically, the random access responseinformation may be transmitted in the form of a MAC PDU (Packet DataUnit), which may be delivered over a PDSCH. To properly receive theinformation delivered over the PDSCH, the UE may monitor the PDCCH usingan RA-RNTI. The RA-RNTI has a value determined based on the PRACH overwhich the random access preamble is transmitted. The PDCCH may containinformation on the UE to receive the PDSCH, and information on frequencyand timing of a radio resource of the PDSCH, and the transmission formatof the PDSCH. Once the UE succeeds in receiving the PDCCH transmittedthereto, the UE may properly receive a random access responsetransmitted over the PDSCH according to the information of the PDCCH.The random access response may include a random access preambleidentifier (RAPID), a UL grant informing of a UL radio resource, atemporary cell radio network temporary identifier (temporary C-RNTI),and a timing advance value. If the random access response includes aRAPID corresponding to the random access preamble which the UE hasselected in Step 1, the UE may acquire a UL grant, a temporary C-RNTIand a timing advance value, considering that it has succeeded inreceiving the random access response.

3. Scheduled Transmission

When the UE receives a valid random access response therefor, the UEprocesses information contained in the random access response. That is,the UE applies the timing advance value and stores the temporary C-RNTI.In addition, the UE transmits, to the eNB, the data stored in the bufferthereof or new data generated using the UL grant. Herein, the dataaccording to the UL grant includes an identifier of the UE. In thecontention-based random access procedure, the eNB cannot determine whichUE(s) perform the random access procedure. To resolve contention later,the eNB needs to identify the UEs. There are two methods for includingthe identifier of a UE in the data transmitted to the eNB in response tothe UL grant. According to one method, a UE possessing a valid cellidentifier allocated in a corresponding cell prior to the random accessprocedure transmits the cell identifier through a UL transmission signalcorresponding to the UL grant. On the other hand, a UE that has not beenassigned a valid cell prior to the random access procedure transmits aunique identifier thereof (e.g., SAE (System Architecture Evolution)TMSI (Temporary Mobile Subscriber Identity) or a random ID). In general,the unique identifier is longer than the cell identifier. The UE havingtransmitted the data corresponding to the UL grant initiates acontention resolution timer (hereinafter, “CR timer”).

4. Resolution of Contention

Once the UE has transmitted, to the eNB, data containing the identifierof the UE in response to the UL grant contained in the random accessresponse, the UE waits for an instruction from the eNB. That is, the UEattempts to receive a PDCCH to receive a specific message from the eNB.There are two methods for receiving the PDCCH. If the identifier of theUE transmitted in response to the UL grant is a cell identifier asdescribed above, the UE attempts to receive a PDCCH using the cellidentifier thereof. If the identifier of the UE transmitted in responseto the UL grant is a unique identifier, the UE may attempt to receive aPDCCH using the temporary C-RNTI contained in the random accessresponse. In the former case, if the UE receives a PDCCH through thecell identifier thereof before the CR timer expires, the UE terminatesthe random access procedure, determining that the random accessprocedure has been normally performed. In the latter case, if the UEreceives a PDCCH through the temporary C-RNTI before the CR timerexpires, the UE checks the data carried on the PDSCH indicated by thePDCCH. If the data carried by the PDSCH contains the unique identifierof the UE, the UE terminates the random access procedure, determiningthat the random access procedure has been normally performed.

The non-contention-based random access procedure may be performedthrough the following steps.

1) Allocation of a Random Access Preamble

The non-contention-based random access procedure may be performed whenthe handover process is performed or the procedure is requestedaccording to a command from the eNB. Of course, the contention-basedrandom access procedure may be performed in both cases. First, the UE isassigned a dedicated random access preamble which will not be subject tocontention in order to perform the non-contention-based random accessprocedure. The random access preamble may be signaled to the UE by theeNB through a handover command or a PDCCH order.

2) Random Access Preamble

The UE transmits the dedicated random access preamble thereof to theeNB.

3) Random Access Response

The UE receives a random access response. The UE receives the randomaccess response from the eNB using the same method as that in thecontention-based random access procedure.

In the random access procedure, the eNB receives the random accesspreamble transmitted from the UE, and calculates the timing advancevalue for shortening or lengthening the transmission timing of the UE,using the reception information of the random access preamble. Then, theeNB informs the UE of the calculated time synchronization value througha random access response, and the UE in turn updates the transmissiontiming using the value. As another method for UL time alignment, asounding reference signal (SRS) may be used. The eNB receives an SRSthat the UE periodically or randomly transmits, and calculates thetiming advance value of the UE through the received signal, informingthe UE of the timing advance value. Thereby, the UE updates thetransmission timing thereof. In a macro cell, the timing advance valuemay vary depending on the position of the UE in the macro cell since thecell radius of the macro cell is large. On the other hand, in a smallcell, the UE may have the same timing advance value of 0 since the cellradius of the small cell is small. In this case, a UE performinghandover or serving cell switch in the small cell performs ULsynchronization for a new serving cell, and thus a process of obtainingthe UL timing advance value of the new serving cell may be skipped.

If there is an order to switch the serving cell of the UE, the UE maytransmit ACK information for the order to a new serving cell to whichthe serving cell is to be switched. In this case, the UE may transmitACK information over a PUSCH of the old serving cell indicated throughan RRC signal, a MAC CE, or an (e)PDCCH and/or the new serving cell.Alternatively, the UE may transmit the ACK information over a predefinedPUCCH of the old serving cell and/or the new serving cell.Alternatively, when the UE receives an order to switch the serving cellof the UE, it may transmit an SRS to the new serving cell. In this case,the UE may transmit information on transmission timing of the SRS andinformation on SRS transmission RB(s) through an RRC signal, a MAC CE,or a (e)PDCCH.

An eNB operating/controlling the serving cell of the UE or an eNBoperating/controlling the Pcell (or the macro cell) of the UE mayinform, through a higher layer signal (e.g., an RRC signal), a MAC CE ora (e)PDCCH, the UE having received the order of serving cell switch ofwhether the UE should perform switch of the serving cell by transmittingan ACK to the new serving cell or by transmitting an SRS to the newserving cell.

If the serving cell of the UE is switched to a specific cell, the eNBoperating/controlling the serving cell of the UE or the eNBoperating/controlling the Pcell (or the macro cell) of the UE mayprovide the UE with information on a non-contention RACH resource forthe UE to use. The C-RNTI which the UE has received from the old servingcell may continue to be used even when the serving cell is switched toanother cell in the small cell set B. If it is necessary to change theC-RNTI, the new serving cell may designate a new C-RNTI. If an RACKresource separate from the RACH resource designated by the old servingcell has not been allocated for the RACH process, the UE may perform theRACH process using the C-RNTI provided in the old serving cell. If thetime at which the UE receives a cell switch command (e.g., a time atwhich the UE receives a PDCCH order) is subframe n, the UE transmits anon-contention-based RACH preamble in subframe n+k (k≧6 in FDD).

If switching of the serving cell occurs, all HARQ buffers may be flushedand a new HARQ soft buffer partition may be designated. If there areScells configured for the UE, the Scells may be maintained in the Pcellswitching process. A previous Pcell or candidate Pcell may inform the UEof whether to maintain or deactivate the Scells configured for the UE.If deactivation of the Scells is signaled, the UE performs thedeactivation process for the Scells.

The eNB operating/controlling the serving cell of the UE or the eNBoperating/controlling the Pcell (or the macro cell) of the UE mayprovide the UE with serving cell switch priority values of the cellsbelonging to the small cell set B of the UE, or provide the UE withinformation on a cell assigned a highest priority for serving cellswitching. The UE may use such information to determine the servingcell. For example, if the UE determines that the UE has beendisconnected from the serving cell as a result of radio link monitoring(RLM), the UE may switch the serving cell to a cell having a servingcell switch priority or having the highest serving cell switch priority.The UE may send an RACH preamble to a cell having the highest priorityto switch of the serving cell, and the cell receiving the RACH preamblemay be configured as the serving cell of the UE without undergoing thehigher layer handover process. In this case, the UE may use anon-contention-based RACH resource allocated to the UE, and the RACHtiming to be applied may be RACH timing of a new cell, namely a targetcell for the UE to access according to the configured information.

Embodiment B OFF State Detection

Each of the cells in the small cell set A may be in an ON state or anOFF state. A cell in the OFF state may not be included in the small cellset B of the UE.

Regarding physical channel transmission, if a specific cell is in theOFF state, this may means that the physical channel of the cell is nottransmitted. Regarding physical signal transmission, if a specific cellis in the OFF state, this may mean that only the CRS/TRS (tracking RS)and/or synchronization signal (SS) of the cell is transmitted with aperiodicity equal to or longer than the current periodicity.Alternatively, if a specific cell is in the OFF state, this may meanthat a new signal called a discovery signal is periodically transmittedin the specific cell in place of physical signals defined in the current3GPP LTE standard.

To correctly perform RRM for the specific cell and to ensure signaltransmission/reception operation in the cell, the UE needs to checkwhether a neighbor cell in/out of the small cell cluster, the cells inthe small cell set A of the UE, or the cells in the small cell set B arein the ON state or in the OFF state. The discovery signal may be asignal in a format identical/similar to that of an existing signal(e.g., a PSS/SSS, a CRS, a TRS) or a signal in a new format. Althoughthe discovery signal is a signal for detecting a cell in the OFF state,it may be transmitted even when the cell is in the ON state.

If the discovery signal is transmitted only in cells in the OFF state,the UE may determine whether the cells are in the ON state or in the OFFstate through blind detection. The UE may determine that a cell in whichthe discovery signal is detected is in the OFF state.

Even when the discovery signal is transmitted not only from a cell inthe OFF state but also from a cell in the ON state, the UE may determinewhether the cells are in the ON state or in the OFF state through blinddetection. Particularly, if a discovery signal transmitted in the OFFstate and a discovery signal transmitted in the ON state use differentsequences or have different transmission periodicities, the UE maydetermine whether the corresponding cells are in the ON/OFF statethrough blind detection. If the discovery signal is transmitted in theON state as well, or if a discovery signal transmitted in the OFF stateand a discovery signal transmitted in the ON state have the sameperiodicity or different periodicities, the OFF state of a cell cannotbe quickly detected through blind detection. To quickly detect the OFFstate in a situation in which the discovery signal is transmitted in theON state as well, a cell ID used to generate a discovery signal for acell in the OFF state may be set differently from a cell ID used togenerate a discovery signal for a cell in the ON state. For example,when it is assumed that there are 504 cell IDs in total, cells in the ONstate may use one of cell IDs from 0 to 251 to generate a discoverysignal, and cells in the OFF state may use one of cell IDs from 252 to503 to generate a discovery signal. In another example, the cells in theON state may use even-numbered cell IDs to generate a discovery signal,and the cells in the OFF state may use odd-numbered cell IDs to generatea discovery signal. The cell IDs given when a cell is in the ON stateand the OFF state may be defined to have a specific relationshiptherebetween. For example, a cell having a cell ID of N in the ON statemay be specified to use a cell ID of N+252 in the OFF state. In anotherexample, a cell having a cell ID of N in the ON state may be specifiedto use a cell ID of N+1 in the OFF state. A separate cell ID(s) which acell can use in the OFF state may be designated or introduced anew.

To allow the UE to readily distinguish a discovery signal from a cell inthe ON state from a discovery signal from a cell in the OFF state, adifferent scrambling sequence may be applied to each discovery signaldepending on whether a corresponding cell in the ON/OFF state. A cellmay apply a specific scrambling sequence to generate a discovery signal,and the UE may use the scrambling sequence to detect the discoverysignal, thereby determining whether the discovery signal is a signaltransmitted from a cell in the ON state or a signal transmitted from acell in the OFF state.

Alternatively, the discovery signal may carry a field, a bit orinformation for indicating whether a cell is in the ON state or in theOFF state.

The UE may obtain information on the cells belonging to the small cellset A from an eNB operating/controlling the serving cell of the UE or aneNB operating/controlling the Pcell (or the macro cell) of the UE. TheUE may recognize whether a specific cell is in the ON state or in theOFF state at a specific time, and obtain information on the transmissiontime of the discovery signal, a cell ID and the like.

Embodiment C RRM in a Small Cell Cluster

The UE may periodically/aperiodically report an RRM result and/or RLMresult for the serving cell, the cells in the small cell set B, thecells in the small cell set A or a neighbor cell to the eNBoperating/controlling the serving cell of the UE or the eNBoperating/controlling the Pcell (or the macro cell) of the UE. The eNBoperating/controlling the serving cell of the UE or the eNBoperating/controlling the macro cell of the UE which is another servingcell of the UE may determine implementation of switching of the servingcell of the UE and a cell to which the serving cell is to be switched,namely the target cell, based on the RRM/RLM result report.

The eNB operating/controlling the serving cell of the UE or the eNBoperating/controlling the Pcell (or the macro cell) of the UE maydetermine switch (or handover) of the serving cell of the UE to cell Bwhich is in the OFF state. In this case, the eNB operating/controllingthe serving cell of the UE or the eNB operating/controlling the Pcell(or the macro cell) of the UE may make a request for switch of cell Bfrom the current OFF state to the ON state to an eNBoperating/controlling cell B. Once cell B enters the ON state, the eNBoperating/controlling the serving cell of the UE or the eNBoperating/controlling the Pcell (or the macro cell) of the UE maycommand the UE to perform switching (handover) of the serving cell tocell B.

Alternatively, the UE may determine to switch the serving cell to a cellin the OFF state. In this case, the UE may transmit a message forrequesting switching of cell B to the ON state to the eNBoperating/controlling the serving cell of the UE, the eNBoperating/controlling the Pcell (or the macro cell) of the UE, or theeNB operating/controlling cell B. If the eNB operating/controlling cellB receives the message from the UE or the eNB of the UE, it may switchcell B from the OFF state to the ON state.

When a cell in the small cell set A, a cell in the small cell set B, ora neighbor cell is in the OFF state, the UE may receive adiscovery/identification signal transmitted from the corresponding cellin the OFF state, and then perform RSRP measurement, RSRQ measurementand the like.

FIG. 14 is a diagram illustrating exemplary transmission of a discoverysignal.

As shown in FIG. 14, when transmission periodicity of the discoverysignal is T_offsig, and a time for which the UE performs RRM in a cellin the small cell set A of the UE, a cell in the small cell set B, or aneighbor cell is T_RRM, T_RRM may be greater than or equal to T_offsig,or may be less than T_offsig. In this case, T_offsig needs to bedetermined within a time range within which synchronization of theserving cell of the UE can be maintained, in order to maintainconnection to the current serving cell while the UE is performing RRM ofa cell in the small cell set A of the UE, a cell in the small cell setB, or a neighbor cell. In this case, to allow the UE to correctlyperform RRM of the cell in the small cell set A of the UE, the cell inthe small cell set B, or the neighbor cell, T_RRM may be set to begreater than or equal to N*T_offsig. Herein, N is greater than 1.

The UE may obtain the discovery signal transmission timing (e.g.,T_offsig used in each cell) for the cell in the small cell set A or thecell in the small cell set B from the eNB operating/controlling theserving cell of the UE, the eNB operating/controlling the Pcell (or themacro cell) of the UE, or an eNB operating/controlling each cell and usethe same to receive a discovery signal.

The eNB operating/controlling the serving cell of the UE or the eNBoperating/controlling the Pcell (or the macro cell) of the UE may informthe UE of information such as the discovery signal transmissiontiming/periodicity and cell ID of a specific cell when it makes arequest for implementation of RRM of a cell in a specific small cell setA, a cell in the small cell set B, or a neighbor cell.

If the UE fails to properly receive a discovery/identification signal ofthe OFF state in a cell in the small cell set A or small cell set B, andthus cannot determine presence of the cell, the UE may inform the eNBoperating/controlling the serving cell of the UE or the eNBoperating/controlling the Pcell (or the macro cell) of the UE of thisfailure. If the cells in the small cell set A or a specific cell in thesmall cell set B is in the OFF state, and the specific cell transmits nosignal in the OFF state, the UE may identify the time at which thespecific cell is switched back to the ON state to determine whether thespecific cell is switched to the ON state at the right time. If thespecific cell is not switched to the ON state at the right time, andthus the UE cannot determine presence of the specific cell, the UE maydeliver corresponding information to the eNB operating/controlling theserving cell of the UE or the eNB operating/controlling the Pcell (orthe macro cell) of the UE.

When the UE reports the RRM result to the eNB operating/controlling theserving cell of the UE or the eNB operating/controlling the Pcell (orthe macro cell) of the UE, the UE may additionally inform the eNB ofwhether the corresponding RRM report is about a cell in the ON state ora cell in the OFF state. More specifically, if the RRM report is about acell in the OFF state, the UE may explicitly transmit informationindicating the RRM of the cell in the OFF state.

A threshold that is used when the UE reports an RRM result for a cell inthe OFF state may be configured or designated through a higher layersignal separately from the threshold which is used when the UE reportsan RRM result for a cell in the ON state. In addition, if the cell isnot from the small cell set A or the small cell set B, or an RRM requestis not received from the serving cell, the UE does not deliver the RRMreport for the cell in the OFF state. For example, if cell 4 is not acell in the small cell set A or the small cell set B, or the UE does notreceive a RRM request for cell 4, the UE does not deliver the RRM reportof the OFF state for cell 4 even in the case when the UE reads thediscovery signal of cell 4 in the OFF state.

When the cell configured for the UE in the small cell set A or the smallcell set B is in the ON state, it may be unnecessary for the UE toconsistently perform RRM for the cell if the UE is out of the coverageof the cell or the channel state is not good. Alternatively, it may beunnecessary to perform RRM for all the cells in the small cell set A asthe small cell set A includes many cells. In this case, the presentinvention proposes that false information indicating that the cell is inthe ON state be delivered to the UE in order to prevent the UE fromperforming RRM for the unnecessary cell in the small cell set A or aneighbor cell. In this case, the UE may not perform RRM for the cellduring a period in which the UE is aware that the cell is in the OFFstate. The false OFF state may be UE-specifically configured.

If a cell transmits a discovery signal in the ON state or the discoverysignal has the same format as that of a channel/signal transmitted inthe ON state, the UE may perform RRM through the discovery signal,assuming that the cell is in the OFF state.

Embodiment D UE ID for a Macro Cell and a Small Cell

In the conventional systems, only cells used at the same location,namely an aggregation of CCs has been considered. However, CCs ofdifferent nodes may also be aggregated. In other words, cellscorresponding to different geographical areas may be aggregated.

A UE ID is configured by the eNB in the random access procedure. Forexample, a C-RNTI may identify a UE in a cell, and may be temporary,semi-persistent or permanent. A temporary C-RNTI may be allocated in atemporary access process, and may become a permanent C-RNTI aftercontention is resolved. A semi-persistent C-RNTI is used to schedulesemi-persistent resources through a PDCCH. The semi-persistent C-RNTI isalso called a semi-persistent scheduling (SPS) C-RNTI. A permanentC-RNTI has a C-RNTI value allocated after contention is resolved in therandom access procedure, and is used to schedule an operating resource.The present invention proposes that the UE use a UE ID of the UE used inthe macro cell and a UE ID of the UE used in the small cell separatelywhen the macro cell and the small cell are used through carrieraggregation.

For example, if the UE ID in the macro cell is separated from the UE IDin the small cell, a specific UE may operate without switching betweenthe UE IDs when the UE using macro cell 1 as the Pcell and small cell 1as the Scell switches the Pcell to macro cell 2 with small cell 1maintained as the Scell. In another example, if the UE ID in the macrocell is separated from the UE ID in the small cell, a specific UE mayoperate without switching the UE ID for the small cell when the UE usingmacro cell 1 as the Pcell and small cell 1 as the Scell switches thePcell to small cell 2 in the cluster of small cell 1.

Embodiment A, Embodiment B, Embodiment C and Embodiment D of the presentinvention may be separately applied or a combination of two or more ofthe embodiments may be applied.

FIG. 15 is a block diagram illustrating elements of a transmittingdevice 10 and a receiving device 20 for implementing the presentinvention.

The transmitting device 10 and the receiving device 20 respectivelyinclude Radio Frequency (RF) units 13 and 23 capable of transmitting andreceiving radio signals carrying information, data, signals, and/ormessages, memories 12 and 22 for storing information related tocommunication in a wireless communication system, and processors 11 and21 operationally connected to elements such as the RF units 13 and 23and the memories 12 and 22 to control the elements and configured tocontrol the memories 12 and 22 and/or the RF units 13 and 23 so that acorresponding device may perform at least one of the above-describedembodiments of the present invention.

The memories 12 and 22 may store programs for processing and controllingthe processors 11 and 21 and may temporarily store input/outputinformation. The memories 12 and 22 may be used as buffers.

The processors 11 and 21 generally control the overall operation ofvarious modules in the transmitting device and the receiving device.Especially, the processors 11 and 21 may perform various controlfunctions to implement the present invention. The processors 11 and 21may be referred to as controllers, microcontrollers, microprocessors, ormicrocomputers. The processors 11 and 21 may be implemented by hardware,firmware, software, or a combination thereof. In a hardwareconfiguration, application specific integrated circuits (ASICs), digitalsignal processors (DSPs), digital signal processing devices (DSPDs),programmable logic devices (PLDs), or field programmable gate arrays(FPGAs) may be included in the processors 11 and 21. Meanwhile, if thepresent invention is implemented using firmware or software, thefirmware or software may be configured to include modules, procedures,functions, etc. performing the functions or operations of the presentinvention. Firmware or software configured to perform the presentinvention may be included in the processors 11 and 21 or stored in thememories 12 and 22 so as to be driven by the processors 11 and 21.

The processor 11 of the transmitting device 10 performs predeterminedcoding and modulation for a signal and/or data scheduled to betransmitted to the outside by the processor 11 or a scheduler connectedwith the processor 11, and then transfers the coded and modulated datato the RF unit 13. For example, the processor 11 converts a data streamto be transmitted into N layer layers through demultiplexing, channelcoding, scrambling, and modulation. The coded data stream is alsoreferred to as a codeword and is equivalent to a transport block whichis a data block provided by a MAC layer. One transport block (TB) iscoded into one codeword and each codeword is transmitted to thereceiving device in the form of one or more layers. For frequencyup-conversion, the RF unit 13 may include an oscillator. The RF unit 13may include Nt (where Nt is a positive integer) transmit antennas.

A signal processing process of the receiving device 20 is the reverse ofthe signal processing process of the transmitting device 10. Undercontrol of the processor 21, the RF unit 23 of the receiving device 20receives radio signals transmitted by the transmitting device 10. The RFunit 23 may include Nr (where Nr is a positive integer) receive antennasand frequency down-converts each signal received through receiveantennas into a baseband signal. The processor 21 decodes anddemodulates the radio signals received through the receive antennas andrestores data that the transmitting device 10 intended to transmit.

The RF units 13 and 23 include one or more antennas. An antenna performsa function for transmitting signals processed by the RF units 13 and 23to the exterior or receiving radio signals from the exterior to transferthe radio signals to the RF units 13 and 23. The antenna may also becalled an antenna port. Each antenna may correspond to one physicalantenna or may be configured by a combination of more than one physicalantenna element. The signal transmitted from each antenna cannot befurther deconstructed by the receiving device 20. An RS transmittedthrough a corresponding antenna defines an antenna from the view pointof the receiving device 20 and enables the receiving device 20 to derivechannel estimation for the antenna, irrespective of whether the channelrepresents a single radio channel from one physical antenna or acomposite channel from a plurality of physical antenna elementsincluding the antenna. That is, an antenna is defined such that achannel carrying a symbol of the antenna can be obtained from a channelcarrying another symbol of the same antenna. An RF unit supporting aMIMO function of transmitting and receiving data using a plurality ofantennas may be connected to two or more antennas.

In the embodiments of the present invention, a UE operates as thetransmitting device 10 in UL and as the receiving device 20 in DL. Inthe embodiments of the present invention, an eNB operates as thereceiving device 20 in UL and as the transmitting device 10 in DL.Hereinafter, a processor, an RF unit, and a memory included in the UEwill be referred to as a UE processor, a UE RF unit, and a UE memory,respectively, and a processor, an RF unit, and a memory included in theeNB will be referred to as an eNB processor, an eNB RF unit, and an eNBmemory, respectively.

The eNB processor of the present invention may control the eNB RF unitto transmit small cell set A configuration information on the small cellset A and small cell set A parameter set information on parameter setscorresponding to the small cell set A according to one of Alternative 1,Alternative 2 and Alternative 3 of Embodiment A. The UE RF unit mayreceive the small cell set A configuration information and the smallcell set A parameter set information and deliver the same to the UEprocessor. The UE processor may store the small cell set A and thecorresponding parameter sets in the UE memory. The eNB processor maycontrol the eNB RF unit to transmit, to the UE, small cell set Bconfiguration information indicating a cell or parameter set of thesmall cell set A which corresponds to the small cell set B. The UE RFunit may receive the small cell set B configuration information, and theeNB may configure a small cell set B based on the small cell set Bconfiguration information. The eNB processor may control the eNB RF unitto transmit indication information indicating a serving cell orparameter set in the small cell set B which the UE actually uses toperform signal transmission/reception. The UE RF unit may receive theindication information. The UE processor may use a parameter setcorresponding to the indication information to cause the UE RF unit toreceive a DL signal or to decode a DL signal received by the UE RF unit.The UE processor uses a parameter set corresponding to the indicationinformation to generate a UL signal or to cause the UE RF unit totransmit a UL signal.

According to Embodiment B in combination with or separately fromEmbodiment A, the eNB processor may control the eNB RF unit to transmita discovery signal through a cell controlled by the eNB processor. TheeNB processor may control the eNB RF unit to transmit the discoverysignal only in a cell in the OFF state or to transmit the discoverysignal in both a cell in the ON state and a cell in the OFF state in amanner that the discovery signal is transmitted with differentsequences, different cell IDs, different periodicities and differentscrambling sequences according to the ON/OFF states of the cells. The UERF unit may receive the discovery signals. The UE processor may attemptto decode the discovery signals, thereby detecting the discovery signaltransmitted from a cell in the OFF state and/or the discovery signaltransmitted from a cell in the ON state.

According to Embodiment C in combination with or separately fromEmbodiment A and/or Embodiment B, the eNB processor may control the eNBRF unit to transmit a message for requesting RRM reporting. The UE RFunit may receive the message. The UE processor may report an RRMmeasurement result for a cell for which a request for RRM reporting hasbeen made in response to the message. The eNB processor requesting RRMreporting for a cell in the OFF state may switch the cell to the ONstate if the cell is a cell controlled by the eNB, or may make a requestfor switching of the cell to the ON state to another eNB if the cell isa cell controlled by another eNB. If the UE RF unit receives a requestfor RRM reporting of the cell in the OFF state, the UE processor maycontrol the UE RF unit to transmit a signal for requesting switching ofthe cell to the ON state. For a cell in the OFF state, the UE processormay perform RRM measurement using the discovery signal (also called anidentification signal) of the cell.

According to Embodiment D in combination with or separately fromEmbodiment A, Embodiment B and/or Embodiment C, the eNB processor maycontrol the eNB RF unit to transmit/receive signals using different UEIDs for a small cell and a macro cell. The UE processor may control theUE RF unit to transmit/receive signals using different UE IDs for thesmall cell and the macro cell.

The embodiments of the present invention may be applied not only to thesmall cells but also to all typical cells. For example, for the typicalcells, the small cell cluster, small cell set A and small cell set Bapplied to the embodiments of the present invention can be replaced witha cell cluster, a cell set A and a cell set B.

As described above, the detailed description of the preferredembodiments of the present invention has been given to enable thoseskilled in the art to implement and practice the invention. Although theinvention has been described with reference to exemplary embodiments,those skilled in the art will appreciate that various modifications andvariations can be made in the present invention without departing fromthe spirit or scope of the invention described in the appended claims.Accordingly, the invention should not be limited to the specificembodiments described herein, but should be accorded the broadest scopeconsistent with the principles and novel features disclosed herein.

INDUSTRIAL APPLICABILITY

The embodiments of the present invention are applicable to an eNB, a UE,or other devices in a wireless communication system.

The invention claimed is:
 1. A method for receiving, by a userequipment, a downlink signal, the method comprising: receiving, by theuser equipment, discovery signal based radio resource measurementinformation for one or more cells; receiving, by the user equipment,discovery signals of the one or more cells at a transmission periodicityof the discovery signals in accordance with the discovery signal basedradio resource measurement information; and performing, by the userequipment, a reference signal received power (RSRP) measurement for theone or more cells based on the discovery signals, wherein the discoverysignal based radio resource measurement information includestransmission timing information of the discover signals, and wherein thetransmission periodicity of the discovery signals is equal to or shorterthan a measurement periodicity at which the RSRP measurement isperformed.
 2. The method according to claim 1, further comprising:receiving first information on a first cell set comprising a pluralityof cells and on a plurality of parameter sets respectively correspondingto the plurality of cells; receiving second information indicating asecond cell set comprising the one or more cells from among theplurality of cells; receiving third information indicating a specificcell in the second cell set; and receiving the downlink signal throughthe specific cell using a parameter set of the specific cell among theplurality of parameter sets based on the third information.
 3. Themethod according to claim 2, wherein each of the plurality of parametersets comprises at least the number of antenna ports of a correspondingcell, zero power channel state information reference signal (CSI-RS)resource configuration information of the corresponding cell,information indicating a start symbol of a physical downlink controlchannel of the corresponding cell or non-zero power CSI-RS resourceconfiguration information of the corresponding cell.
 4. The methodaccording to claim 2, further comprising: receiving fourth informationindicating switching of a serving cell to another cell different fromthe specific cell through the physical downlink control channel of thespecific cell; and receiving another downlink signal through the anothercell using a parameter set of the another cell among the plurality ofparameter sets based on the fourth information, wherein the another cellbelongs to the second cell set.
 5. The method according to claim 2,further comprising: attempting to decode a discovery signal for eachcell in the second cell set and determining a state of each cell in thesecond cell set.
 6. The method according to claim 1, wherein thediscovery signals of the one or more cells are non-zero power channelstate information reference signals of the one or more cells.
 7. A userequipment for receiving a downlink signals, the user equipmentcomprising: a radio frequency (RF) unit; and a processor configured tocontrol the RF unit, the processor further configured to: control the RFunit to receive discovery signal based radio resource measurementinformation for one or more cells, control the RF unit to receivediscovery signals of the one or more cells at a transmission periodicityof the discovery signals in accordance with the discovery signal basedradio resource measurement information, and perform a reference signalreceived power (RSRP) measurement for the one or more cells based on thediscovery signals, wherein the discovery signal based radio resourcemeasurement information includes transmission timing information of thediscovery signals, and wherein the transmission periodicity of thediscovery signals is equal to or shorter than a measurement periodicityat which the RSRP measurement is performed.
 8. The user equipmentaccording to claim 7, wherein the processor is further configured to:control the RF unit to receive first information on a first cell setcomprising a plurality of cells and on a plurality of parameter setsrespectively corresponding to the plurality of cells, second informationindicating a second cell set comprising the one or more cells from amongthe plurality of cells, and third information indicating a specific cellin the second cell set, and control the RF unit to receive the downlinksignal through the specific cell using a parameter set of the specificcell among the plurality of parameter sets based on the thirdinformation.
 9. The user equipment according to claim 8, wherein each ofthe plurality of parameter sets comprises at least the number of antennaports of a corresponding cell, zero power channel state informationreference signal (CSI-RS) resource configuration information of thecorresponding cell, information indicating a start symbol of a physicaldownlink control channel of the corresponding cell or non-zero powerCSI-RS resource configuration information of the corresponding cell. 10.The user equipment according to claim 8, wherein the processor isfurther configured to: control the RF unit to receive fourth informationindicating switching of a serving cell to another cell different fromthe specific cell through the physical downlink control channel of thespecific cell, and control the RF unit to receive another downlinksignal through the another cell using a parameter set of the anothercell among the plurality of parameter sets based on the fourthinformation, and wherein the another cell belongs to the second cellset.
 11. The user equipment according to claim 8, wherein the processoris further configured to attempt to decode a discovery signal for eachcell in the second cell set.
 12. The user equipment according to claim7, wherein the discovery signals of the one or more cells are non-zeropower channel state information reference signals of the one or morecells.
 13. A method for transmitting, by a base station, a downlinksignals, the method comprising: transmitting, by the base station,discovery signal based radio resource measurement information for one ormore cells; transmitting, by the base station, discovery signals of atleast one of the one or more cells at a transmission periodicity of thediscovery signals in accordance with the discovery signal based radioresource measurement information; and receiving, by the base station,information on a reference signal received power (RSRP) measurementperformed by a user equipment for the one or more cells, wherein thediscovery signal based radio resource measurement information includestransmission timing information of the discovery signals, and whereinthe transmission periodicity of the discovery signals is equal to orshorter than a measurement periodicity at which the RSRP measurement isperformed.
 14. The method according to claim 13, further comprising:transmitting, by the base station, first information on a first cell setcomprising a plurality of cells and on a plurality of parameter setsrespectively corresponding to the plurality of cells; transmitting, bythe base station, second information indicating a second cell setcomprising the one or more cells from among the plurality of cells;transmitting, by the base station, third information indicating aspecific cell in the second cell set; and transmitting, by the basestation, the downlink signal through the specific cell using a parameterset of the specific cell among the plurality of parameter sets based onthe third information.
 15. The method according to claim 13, wherein thediscovery signals of the one or more cells are non-zero power channelstate information reference signals of the one or more cells.
 16. A basestation for transmitting a downlink signal, the base station comprising:a radio frequency (RF) unit; and a processor configured to control theRF unit, the processor further configured to: control the RF unit totransmit discovery signal based radio resource measurement informationfor one or more cells, control the RF unit to transmit discovery signalsof at least one of the one or more cells at a transmission periodicityof the discovery signals in accordance with the discovery signal basedradio resource measurement information, and control the RF unit toreceive information on a reference signal received power (RSRP)measurement performed by a user equipment for the one or more cells,wherein the discovery signal based radio resource measurementinformation includes transmission timing information of the discoverysignals, and wherein the transmission periodicity of the discoverysignals is equal to or shorter than a measurement periodicity at whichthe RSRP measurement is performed.
 17. The base station according toclaim 16, wherein the processor is further configured to: control the RFunit to transmit first information on a first cell set comprising aplurality of cells and on a plurality of parameter sets respectivelycorresponding to the plurality of cells, second information indicating asecond cell set comprising the one or more cells from among theplurality of cells, and third information indicating a specific cell inthe second cell set, and control the RF unit to transmit the downlinksignal through the specific cell using a parameter set of the specificcell among the plurality of parameter sets based on the thirdinformation.
 18. The base station according to claim 16, wherein thediscovery signals of the one or more cells are non-zero power channelstate information reference signals of the one or more cells.